It's not just the cost of the unit. There are plenty of aircraft in the UK with no electrical system and not just gliders. They rely on hand held radios or a lightweight battery to power a simple installation. There are very few transponders that draw sufficiently low power to operate with this kind of system let alone Mode S ones.
I have seen a Tiger Moth with a transponder fitted but it was very much the exception in this class of aeroplane.
As I understand the current situation approximately 50% of registered aircraft in the UK are microlights. Where the hell do you fit a transponder on a weightshift machine?
