As others have cautioned, no talk on the site message system (or in your profile) of PPM, or sex, or anything that could be considered any form of "quid pro quo" (this for that) offer to pay for sex. Even if she offers or asks, do not answer or directly reply. Just suggest that it will be easier (or faster, more convenient, etc.) to move to text, whatsapp, snapchat, or anything off site to discuss details. No problem giving her a phone number or your app user ID.
When I get her to offline mode, I generally ask a few questions to get a sense of her, then I direct the convo towards a M&G. I really want a M&G to discuss all details of the arrangement before any talk of allowance comes up. But if she starts pressing on allowance - either in text or face to face - I ask her to tell me about her last arrangement (I don't ask about allowance - yet). Questions like "tell me what you liked most about your last arrangement?" often gets you a ton of useful info. She will likely also tell you what she didn't like. And then you can respond and reinforce that all the things she liked are things you like and all the things she didn't like are things you would NEVER do. She may go on to tell you what her allowance was, or you can prompt it by asking if she was happy with her allowance. It's important to keep the discussion about her last arrangement - not your potential new arrangement - until you are ready to do so. Once you have an idea of her expectations (remember total allowance = PPM times # visits per week or month) you can then let her know what allowance you have typically provided in your best past arrangements.
My suggestion is to frame your number as your "budget." For example, "My usual budget for my last arrangement was $300 each time we met" (don't use terms like "PPM" even if she does). If your number matches hers, it's all good. If your number is less than hers, you can let her do the math and decide if she wants to accept your implied offer or counter. Either way you have acknowledged her number (i.e.: " what she thinks she's worth), without implying you don't agree. And you have provided her a number that relates to your ability/willingness to pay, and not related to her worth. Every woman's "worth" is astronomically high ("priceless"). Just ask her! Trying to negotiate based on her worth will be a losing strategy. But negotiating based on your budget is a safe way to control your costs without offending her sense of worth. If she likes you, she will likely accept your budget, or will be ok with meeting you in middle. So her ask of $500 versus you budget of $300 may end up with an agreement at $325 to $400.
When I get a POT who insists on discussing PPM before anything else, I assume it's already going to end poorly. In that case I will generally just give her my mid-range number, again as my "budget" and see what she says. I really hate to go this way as it makes it much harder to then discuss or negotiate any specific activities I want with her, like oral, BCD time and date time/length, etc. Each activity I may want (like a 5-8 hour date versus a 1-2 hour slam and go) may result
in her asking for more. Then I'm just negotiating against myself, and the process will just spiral down the drain.
There are a lot of posts here about negotiating strategies. I recommend you read them. Use search terms like allowance, negotiate, and offer to find them.
Life is good
The Cat