I recall news stories in which chicks were charged with prostitution for exchanging sex for valuables other than money.  It's the direct quid pro quo that is illegal.  Any sort of relationship muddies the waters as far as quid pro quo is concerned.  So sugar daddy/sugar baby relationships are mostly immune from the law.   The risk is always in the sting operation where you are meeting someone for the first time that you don't know and offer money for sex.  Just about everything else is too muddy to untangle for purposes of prosecution.  

So SA really has a low bar to obtain in order to stay "legal."  As long as they openly prohibit p4p they can benefit from wink wink relationships.

PPM should not be used in SA messaging to avoid the SA admins flagging you. I mostly suggest a weekly allowance to begin meeting once or twice a week and then moving to a monthly allowance. Once she starts texting you privately then you can talk numbers if you really have to, I try to meet them first before promising a specific amount.

I've been in the bowl about a month or so.
I documented it.
Many know that I carried a badge for a living in a major city many moons ago.
100% guaranteed I've come across at least three possible LE types playing in the SB bowl as a trap.
Knowing what to look for comes in handy.
My recommendation is to not be too full of yourself, pay attention to verbiage and be more explicit in your qualifying questions
Don't be afraid to ask for intimate pictures. If they refuse dissect their reasoning.

Offered an SB cash for sex.  Not in SA Messages, not in text, not even face to face in public.  
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When talking to a POT SB, I generally describe past SB arrangements that typically included some public activity, some private time at my place and at the conclusion of our time together, an offer to help her cover some bills or expenses. It's always in the context of a time spent on a date, doing date-ish things.  This is not a payment. It's an allowance, and when I hand it to her I usually call it a present.  
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Once we have agreed to the date context (and off SA messages), we might discuss the activities (both public and private) in more detail.  I see no difference between talking about what type of movies and food we like and talking about what each of us likes sexually. There's nothing there that doesn't happen in "civvy" dating. Note that I will never affirm or directly reply to any "quid pro' quo" statement she makes. I'll change the topic or deflect with a different context.

It's never pay per meet...

It's an incremental payment of a monthly allowance, paid in aperiodic installments.

I almost always refer to payments as gifts or favors because it really just sounds better and has less of a wage like connotation.   This is important if you want to talk her down or if you end up really liking her and you want to spend lots of time with her for just taking her on vacation or to a nice dinner.  I've landed pro sugar babies who usually get way more per hour but I was able to get them to think of me as good, safe, sex that comes with with a free gift and good conversation rather than just another john who has to pay by the hour.  Every little thing helps and I wouldn't dismiss this as splitting hairs.
 
I also agree that an in person meet and greet is the best way to get around all of that prostitution stigma.  Not only for LE purposes, but also to make the girl forget about the whole prostitute thing.  If she says, "I'm not a prostitute." or anything along those lines during the meet and greet or while scheduling it, all you have to say is, "Of course not! Did you think I would take a prostitute out for coffee?  haha"