So we agreed to meet on Monday night at 7. I landed at 5:30 and just had enough time to check in to my hotel and drive the rental to the bar/restaurant. As I got near the hotel she texts and asks if we can delay to 8 pm. I agree and now have an extra hour at the hotel.  
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I leave for the bar with enough time to be 15 minutes early (as usual) and arrive at 7:45. She's not there yet and I order a glass of wine and start to text her that I'm early. As I'm typing she texts me that she's running late and wants to try 10 pm. That's too late for me. I have early meetings in the AM so I tell her no and how about tomorrow?  She agrees on 7 pm same place and says "I'll make it up to you". (Never heard THAT before! I head back to the hotel, out of pocket $24 for a glass of wine and valet parking, and an hour round trip travel time. Stop at an In-n-Out on the way back and enjoy my Double-Double - alone.  
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Next night I text a confirmation at 5:30 and get a positive response.  Get to the bar at 6:45 and she texts at 6:58 that she's parking. Took her a few to find the place (located in an outdoor upscale mall) and we sat down at the bar at 7:05. All seemed well. She's sweet, slim, a bit shy and very, very cute. Me LIKEY! We chat about various topics and eventually I steer the convo to why she's on the site, etc. She says she's new - well she was on a while ago for a few days but stopped, then restarted a few weeks ago. Says I'm the first meet she's had from the site.  We keep chatting and I ask if she'd like dinner. Yes. We order at the bar and keep chatting. Now she opens up a bit. She's actually had 2 prior arrangements. The fist was not great, but the second ended recently on good terms. He was giving her a monthly allowance and  at first they would meet weekly... then the tempo slowed as he "got more busy" and the meets went down to once or less a month. He eventually ended it as he "just didn't have enough time" for her.  So now she is hoping to get a new SD with the same arrangement.  
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I'll pause here for two observations:  1st - She said she was essentially new on the site, but had 2 previous SD's. So she's 21 now, barely. That means she had SD's as early as 19.  2nd- She said the reason she was delayed the night before was that she had just moved to the "area" that day  - from a city 145 miles away. That means she was in the other city when she reactivated her profile.   And her new location was still 25 miles north of the city on her profile.  Two (I'll be polite and call them) conflicts that were not really explained.  
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Now dinner progresses and I dig for more info.  How much was the monthly allowance?  She held up her napkin to her mouth so the bartender presumably couldn't see her and said $5,000 a month.  
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Silence.  
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More silence.  
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Finally, I said "Do you still have his number?" She did. I said "Call him - now. Or, give it to me and I'll date him for $5 grand a month."  I was smiling while I said it, but I was thinking "WTF kind of idiot pays $5k a month, especially for an SB who literally lives in a low-rent market in the stix?"  She laughed, and I thought she got the hint that I will not be that SD. She did not.  
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As we finished the food, I let her know that I'm only going to be in the area every 4 to 6 weeks and would only offer her a per meet allowance.  She seemed ok, but of course we had not discussed the actual number. I told her we would not be going BCD that night, so we needed to work out what happens on my next visit. I paid the check and we went outside to be away from other people.  We got into details of what a "meet" would look like; time together, activities out and in doors etc. We seemed to be in agreement on the details and she asked me how much. I went with the "here's what my last arrangement was like" line and offered $300-$400 per. She countered with "can you do 5?" I thought about it; she was hella cute and submissive, seemed down for anything and everything I wanted sexually and I'd only see her infrequently. So I agreed on $5.  She seemed happy and I asked her if she had a recent test.  That's where it all fell apart over the next 10 minutes.  
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As soon as I asked about testing and birth control she said she'd need more to accommodate the "extra effort". Her argument was that it's more time and effort for her, plus more expense. I countered that if she had to pay for either of these through her current health plan I'd find her a local family planning clinic and cover the direct costs, but I'm sticking with the five hundred per meet. Her jaw dropped and she said "five hundred? I thought you meant five thousand a month?" I quickly made it clear that $5k was NOT going to happen. It's $500 per meet. She countered at $700 (for the "extra effort") and said she could not go lower.  And that's where it ended. I told her we should each take a day or two to think it over and the we can reconnect and talk some more. Gave her a nice card with the promised $40 and a hug. Then said goodnight and headed back to my rental car.  
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Final position: This deal is dead. I won't re-contact her. If she contacts me and agrees to my offer, I'm back in.  
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Clearly, I did not handle this perfectly. I ignored the early and ongoing red flags and tried to salvage a good deal from a poor starting position. It certainly did not help that some fucktard happily agreed to pony up $5k a month for less than weekly meets (assuming that was not an outright lie). But I could have stopped at the 1st drink and bailed without the $135 dinner check.  Another Double-Double would have been just $4.95 Little Head tried to win the day, and both Little Head and Big Head lost.  

The problem isn't her signing some unenforceable, uncollectable promissory note. The issue is what can I do with the note besides wipe my ass with it when she ghosts on me?

 
I have a couple of pretty iron clad rules of thumb where it comes to money and the women I have sex with. I wish I could say I had an iron clad rule about NEVER advancing money under any circumstances, but I am not quite that rigid. HOWEVER I never advance even a single penny to a person I have never met, I only will give gas, uber, or maybe babysitting money to a girl I have never had sex with, and I will only advance the amount of money she can "earn" back in a single session to a girl I have known  less than a year or have slept with  less than a couple of dozen times.  After that, it's on a case by case basis, but that happens so rarely it's hardly even worth discussing. lol