Negotiating should always be based on your budget, not her "worth." Her "worth" is a subjective, ambiguous amount. How much is she "worth" at any given time? She is priceless! Just ask her (because her opinion is all you have)?
On the other hand, your budget is an objective, measurable amount. It's your actual budget, and she can't really argue that.
I am assuming you just don't want to go with $800 with same frequency (# BCD's/month) as other SB's.
So first, you can try the money over time model:
If your budget is $500 3x a month, that's $1,500 a month assuming you consistently meet your schedule. So if you don't think she will come down from $800, then stretch the proposed schedule to 2x a month for a total of $1,600 or meet once every 3 weeks for an average of $1,200 a month over a 3-month period. This works best with SB's who are all about the money (vs. luxury lifestyle). It works even better for SB's that are, or will be, dating multiple SD's, as she will fill up the downtime with other old, wrinkled penises.
Next, you can try the value vs price model:
Tell her this: "If we can agree on $500 per meeting, I will be able to provide more exceptional experiences for us. We can go to more nice restaurants, travel more often, I can buy you better gifts, etc." This works best with SB's who are equally interested in all the upscale lifestyle events you can provide as well as, or more than, the money.
Bottom line: Do not offer $1 more than you are comfortable offering. That will eventually erode any satisfaction you will get from this arrangement. Neither of the 2 models above are perfect, but they should serve as a guideline to get you closer to your goal of enjoying that sweet, box-checking pussy.
Life is good
The Cat