That's near impossible to answer because the variables are huge. But I would say a reasonable range that covers most I've seen is 200-600 for a meet. It varies by how long the meet, what we're planning to do, and the biggest variable is the woman herself. They vary a lot on how much of it is about the $$$ for them. Some have been more interested in advice/mentoring/a good lead on a position with a good reference.
I helped got one an internship at a competitive small company, and mentored her for the year she was there. At the end of the year she was hired to a regular position with them. I saw her once or twice a month for the year, many overnights, and the direct cost was very minimal. Probably the last six months totaled $500, almost all in extra presents (birthday, etc.). It isn't always about the size of the cash donation.
I haven't done many on a monthly allowance. One at $700/mo, one at 1000. The first one lasted a long time, the second not very long (it didn't end because of the money). Guys I know with more experience with monthly SA situations say anything 800-2000 is common. Some are unrealistically asking 5K. Some who are new to the game can be found as little as a car payment each month. As with PPMs a lot depends on her--looks, expectation of frequency, etc.