This is basically a commission for executing a part of the sales process, so the first suggestion is that only appointments that result in closed sales result in a commission (even though the appointment setter doesn’t close it). That will lead to better quality appointments and some accountability for not wasting the time of the “closers.”
As far as how much to pay, it’s probably best to start with the total comp needed to attract the right kind of people into the role, and a reasonable number of appointments they should be able to set (that close) per month. If some portion is being paid as base salary, then take the remaining total comp at target per month and divide by the expected number of total appointments that close, and that’s the rate per appointment. Finally, check affordability – is each appointment worth that amount to the company? If not, the sales model doesn’t work and needs a re-think. If so, you’ve got your answer.
The rate per appointment varies wildly from one industry to the next, based on the maturity of the business, the strength of the brand, access to good target buyer lists, etc. But the fundamentals above will guide the comp designer to the right answer.
I have put in links to a couple of “Sales Comp Answers” below that are not specifically about this exact situation, but that do give more detail about the principles that will guide you here.