Kathleen,
It's a given that people love a behind-the-scenes collections storage tours, but obviously they present some challenges. The logistics of moving people through what are generally tight and carefully arranged spaces can be problematic so it's essential that there be a thoughtful discussion with the group prior to entering your spaces about what boundaries or rules are in place--even then after laying down some guidelines, I have still had tour-goers bring drinks in, or disregard taped boundaries on the floor, or forget to leave their coats/bags in a designated space, or take notes with pens, etc. These are things, that for collections folks, are usually intuitive but require considerable monitoring with a public group (many of these things even happened to me during a tour lead for colleagues who worked in other departments). And ultimately the number of guests is really determined by the size and layout of your space and how many people are leading or facilitating the tour, though what previous posters have said, between 10-20ish seems about right.
In regards to cost, I'm generally not keen on the idea of charging for collections tours. I understand that it requires staff time to prepare the spaces for the public and to conduct the tours, and that that time has value, but I don't like the idea of that access to works in museum collections being cost prohibitive; in a lot of cases admission to the museum alone can be out of reach and special access in the form of a collections tour, in my opinion, deepens the divide between the haves and the have-nots. Thus I'd be inclined to allow free collections tours to only research groups, students, etc. who will have some direct education benefit.
Call me idealistic, but one of the core tenets to working with collections is to perform our work without bias and I think once you start bringing in other departments (finance/events/guest services) by way of public tours for profit, it sets a precedent whereby the potential for profit might begin to dictate the way a collections department would operate and may give those interests too much of a stake. I know firsthand that working in a collection can often feel like working a silo, but sometimes for good reason..
Just my two cents,
Erica