There are many posts here on negotiating. Please search the forum for those as they provide excellent advice. But here's a quick recap:
Asking you what is "fair" is essentially asking you what she is "worth." And her "worth" is a slippery slope to be avoided.
Several problems with "fair" or "worth":
1. It reinforces the transactional context; you must pay a fair price for her service.
2. This is way too subjective and can (will) end badly for you. What is a woman "worth"? She's priceless! Just ask her!
3. IMPORTANT: In the "fair" or "worth" context, SHE has control and has leverage on your offer. If you offer $400, she can counter with "Am I not worth just a hundred more than that?" If you say no, you just insulted her and that is never a good idea.
If you negotiate from your budget, YOU are in control. Any arguments of "I'm worth more" are moot. Sure, she is indeed worth millions just to be in her presence! My suggested negotiation strategy is to base your offers on YOUR budget. This is objective and stays away from "fair" and "worth." Your budget is your budget. Asking you to go over your budget risks her not getting ongoing dates, or dates at the frequency you can budget. So I use language like this:
"My budget is around $300-$400 whenever we meet. Plus, I will always pay for all expenses during our date of course, like dinner, tickets, travel, and such. How does that sound to you?"
If she counters with a higher number, you need to make a decision if you want to accept it or recounter. For example, if you are willing to go higher, you can take the expensive dinners off the table and shift those funds into your allowance budget. (She may even reply that she's not really looking to go out to dinners and such. That makes it easier.)
Finally, be prepared to WALK AWAY if you can't agree on a comfortable number. If you are looking for an ongoing arrangement, going 20-50% over your budget (budget = rate including incidental expenses like dinner and/or hotel room x # meetings per month) you will find yourself getting less satisfaction with her over time as you will always be overpaying. If you are just looking for a Bang and Bail (BAB) because she is insanely hot and a total freak in bed, then going overbudget for one or two BCD's may be a good investment.
Alos, keep walking away as a viable option: There will always be more POT's. Always. I did a "back of envelope" math problem a few years back using US Census data. By my rough estimate, there are around 1,000 to 20,000 women turning over 18 EVERY month in EVERY state in the Union.
Life is good
The Cat