That's what I said. The only real cost associated with airtime are the cost of 50-hour inspections and such. At my club, these are done by volunteers, so there's no direct monetary cost.
The only other thing I could possibly think of, that is directly related to airtime is any "lifed" items, that need to be replaced/overhauled/whatever after x hours of airtime. This is quite common in the powered world, but I have no idea if there are any gliders that have the same issues.