Given that my favourite form of aviation is any glider built 40+ years ago,is made of wood, preferably doesn't have a cockpit canopy and is certainly valued at less than £2k by the insurers, I would regard "expensive" as anything much more than £200.
It would also need to be easily moved from one aircraft to another 'cos I wouldn't want to buy a transponder for each one I fly!
So something akin to a cheap GPS or I join the "antis" very smartly!
PS as I understand it, if everything had a transponder, ATC would then have to install software to screen out most of the slow moving "clutter" - so why put it in to start with?