The
big problem with transponders in gliders and other a/c without a generator/alternator is the heavy current drain. A glider battery fits in your hand.
There's some 5000 Flarms installed in European gliders. They cost some 650 Euros and only require 52mA.
http://www.flarm.com/product/index_en.html
This is a
far more practical way to go for g/a than mode S transponders for everybody, especially considering how long it will be before mode S capable radars are put into operation -- 20 years or so.