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. If it hasn't then perhaps look for a 'package deal' from the same supplier/manufacturer at about £2800.
How receptive your syndicate partners are likely to be rather depends on the sort of flying they prefer to do.
Last edited by ASRAAMTOO; 7th November 2019 at 15:35.