I think $25 is reasonable as a deposit for a nationally known instructor. For a local member in a local setting a lower amount may be reasonable, such as $5 to $10. There may be fees for the group/room that you don't know about, or donations to the location that are made/expected. If it's at a church or school, there may be people who have to work late/on another day to allow you to be there. In any case, there would be a limited number of spaces, so they would have some minimum number of people in mind, and a known maximum.