If your new galleries only stay for a certain amount of time then you could just charge per spot. bunnyteens does it this way, and the price has been slowly creeping up to match the demand... it was $7 about a year ago, now it's $20. I presume he nudges it every time the paid submissions tray starts overflowing.
You would probably make a lot more in the long run with individual submissions for $ rather than partner accounts, since it's a much fairer way to match the amount of galleries that the webmaster will be submitting over a month. You could still require new webmasters to show you some sample galleries before you accept any submissions to keep the quality high.