Originally Posted by
ASRAAMTOO
Expect to pay about £1700 list price for a transponder plus fitting. You say its a permit aircraft so you may be able to do this yourselves. A modern transponder will not be too much of a drain on the battery although its possible your existing radio may be, unless its been replaced recemtly.
Radio is an 8.33 so I believe it's been replaced recently, though can't remember what brand. Do you know how much roughly it would cost? It's an LAA permit to fly.