I would ask what is it about gliders that make them different to other aircraft such that they shouldn't carry a transponder when powered aircraft must at FL100. A glider is just a much a collision risk as any other aircraft. If it had a transponder then there is a much better chance that an ACAS equipped aircraft would be able to avoid.
I do however share sympathy for cost - I have a number of family and friends who have spent time on gliders - a transponder is a little pricey compared to the cost of purchasing and running a glider. Having said that, there are a number of new boys on the scene - Trig, Filser(*) to name a couple who come to mind - who are making reasonably priced lightweight Mode S transponders. It could be possible to get a reasonable glider sized piece of kit not too far in the future.
RS
(*) other transponder manufacturers are available
