I have had success with SB at 200 to 250.  I finally get one lately  on the line and she wants 400.  I agree after much agonizing and then she wants 500.  Another one is at 350, ok I agree.  Before we meet she goes for a bump, Insane!!  
I’m out of my comfort zone over 300 and then they want more.  I’ll just handle it my self.  

This is the flip side of past discussions of SD's who want to negotiate down after one or more BCD sessions. Once a deal is set, it's usually a losing strategy to try to renegotiate a more favorable deal for yourself, whether you are an SD or an SB.  
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Despite, or perhaps because of, the COVID crisis, sugaring remains an endless exercise in patience and repetition. I'd have either stepped back to my line of "this is my budget" that's all I can (never say "will") offer, or if I thought she was just playing me for a sucker who won't risk losing her Golden Pussy, I'd just pass with a "No, thank you. Good luck in your search."  The key is to listen more than you talk.  
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Once she realizes her attempt has failed, she will either come back to the original agreement, or will move on and look for new target she can play for a chump.
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I just landed  a new 22 year old Latina spinner who actually talked herself into a low ball offer. I was almost drooling during our M&G at my home, and was prepared to go as high as $400 if necessary. But when the topic of allowance came up and I asked her what she needs (note: Never ask what she WANTS), she said she's looking for $300 for overnight, and presented test results, confirmed she's on birth control (showed her Planned Parenthood screen) and hates condoms.  Then, as I simply listened and she drank another sip from her 3rd glass of wine (Cline Cellars 2017 Grenache - yumm!) she went on to tell me she only expects allowance IF we go BCD.  I take a moment to "consider" (ok, gloat about) her offer before accepting it. Then led her to my shower for some soapy pre-game fun before heading to my bedroom.  She took instructions well and after I'd spent a fair amount of time delivering 3 to 5 O's, begged Daddy to fuck her.  Later as I helped her to my door to say goodnight, I told her I'd added some extra to cover her Uber ride.  She looked delightfully surprised at my generosity. Because...
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Life is good.
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The Cat

I had a girl, who after moving communication from SA's PM to texting, ask for $1K. I told her, "I don't go above $400 or so, and I understand if you need to stick with $1K and good luck with your search." She came back at $600, and we agreed on that. We had had some extensive texting, and though I thought she was a bit of a butter face, she was game to go BBFS so long as I pulled out. Then as it came closer to meeting, she wanted to confirm that I was giving her $1K, and I had to send her a screenshot of our discussion about $600, and she was good with that. So the night she came over, we got food to go, had a casual dinner which she enjoyed (and I told her I'd send the leftovers to her to take home to Mom and Sis), did our BCD activities which started with a romantic bubble bath and teasing, then on to the bedroom. After we finished, I laid out six Benjamins on the nightstand while she was showering and I went to the kitchen to pack up the food. She comes out with the six Benjamins saying we agreed on $1K. The last thing I wanted was any drama, and we did have a good time, so I gave her the other 4 and sent her on her way... she wants to know when we're getting together again as she loved the romantic bath we took and the 6 orgasms I gave her... well... that would be never. Poor thing hasn't a clue.

Have been talking with a girl to meet, we have talked couple of times to meet.
Just today she contacted me last night to see if I was interested in meeting, told her yes
and asked if tonight was good, she said yes.  

 When I messaged her about 2 hours before time to meet
she replies, was that for tonight?  Told her yes, we talked earlier today.  She said, sorry I guess I must have read it wrong.
So I tell her if she changes her mind and is serious about meeting for an arrangement to get in touch,  have a good night.
She replies that she said yes, another time.    

So either I am missing something, she is a crappy communicator, or she sent a message I never got.
I am going with B, crappy communicator.  Does not talk much, if she sends a whole sentence that is a lot...
Chick is a flake but want to keep the door open to bang the crap out of her as she is young, petite and tight.
She is also only looking to do fwb, nothing more.  Just meet, get money,and have sex, wants nothing else
She is a college girl but is pretty much a semi pro whore now compared to when she first started two years ago.

Friend filled me in on how innocent she was then compared to now as he still bangs her, now she is just an outright whore in his words.  lol

I wouldn't be surprised if she was just messaging multiple guys at one time and, as might be expected, didn't bother to take notes on what she said to each or what they said to her.  
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If I am correct, her primary objective is to get at least one guy to agree to meet "right now" or as soon as possible. If she gets more than one "yes" she goes with the most promising ($$) guy. If he drops out, she moves on. When she said "sorry I guess I must have read it wrong" she means she blew you off for a more promising guy.  
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I'd suggest you move on. If/when you bubble up to the top of her "tonight" list, you'll probably get hit with a giant GPS ask.  
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Life is good
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The C at

She fucked you in the ass and now you need to get a foot shoved up there again.  
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Bad Daddy! Bad!
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You already checked her once by showing her the screen shots. Why didn't you do so again? I get that you wanted to avoid any drama, but there are ways to handle this:  
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1. Remind her again that she agreed to $600, and tell her that's what she's getting.  
2. Tell her you really like her and think you can up your game to $1k NEXT time. Then ghost her.  
3. Smile BIG and tell her for the extra $400 you need some morning sex, then lunch time sex and then she can clean your house and do your laundry - just like your ex wife used to do.  Then laugh like it's joke.. but it isn't.  
4. Tell her the extra is in the doggy bag and take her outside before you hand it to her (without the cash) and then close and lock the door.  
5. Just stare at her and say nothing... nothing... for at least 2 minutes. She will eventually say something and then you can tell her you are disappointed, or surprised, or confused and sorry, maybe next time.
6. Flip the "screen shot" script and ask her look at her phone for your last 4 text messages....  
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On a minor note... Dude?  $600?  even for BBFS BCD that's on the high end of the range. Unless you are in a crazy market like Washington DC. Are you?  
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Life is good.
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The Cat

Recently cut off communications with a young hot POT who made it through all of my screening criteria because every response from her was "Yeah" or "OK".

Me:  Here is my schedule for next week, would you like to meet for dinner and drinks"
Her:  Yeah

Me:  How was your weekend?  I ......and ..... and.....
Her:  Fine

Thank you, next!

believe me, the opposite pole is worse.   Much, much worse.

I can't imagine that I would have given her the extra $400 when you both agreed to, twice, $600.  I would have brought up the text message again.  I mean a deal is a deal, right?  At least you had a good time with her, but that didn't warrant, to me,  giving the extra $400 because she has a bad memory.  Or, maybe she just wanted to see what you would say/do.

Actually we talked about her gift already  spend 2-3 hours at hotel together $300  She made that offer to me.
Which I find reasonable.  

If I understand what you mean by the opposite pole I'd have to agree.  I had a nice arrangement with a young blond cutie a few years ago who simply Would. Not. Shut. The. Fuck. UP!  God could that girl talk, and always about people and events I was not interested in.  I finally gave up on her because of her mouth, even though in other respects her mouth was top notch.

Herb is right as usual, but don't feel too bad about how it played out. Most all of us have had a similar experience. Just promise to learn from it.  I was chasing a young cutie a few years back and we finally agreed on 300.  I mean, several messages and negotiations and we agreed on 300, no doubt in my mind whatsoever.  She shows up and claims it's 600 if I want intimacy.  Major upsell, I let the little head take over and I paid her ask.  I've never and will never do that again!  So I learned a lesson and education is expensive.  Just don't be the idiot who pays for the education but learns nothing from sitting through the classes!

Typically, more like a Chinese or Russian major!  lol
Had a number of these, you know, less than 5 word sentences - when I played along with them (keeping my response to 4-5 words) they promptly dropped away.  Strangely, the past two have not come on SA since, so it's definitely some type of scam.