Mode s transponders start at around £1700 inc vat. Under LAA rules the unit is tested once a year at permit renewal (and on instalation). It is relativly simple to fit one yourself (on permit aircraft) so fitting cost is zero except for the inspection.
Fitting one to a c of a aircraft depends on a lot of things but ranges from around £400 to £5000!
There are a few plug compatable units as slot-in replacments for the KT76a/c. Typical cost is around £1900 inc vat. With this you simply slide out the old unit and slide in the new!
Rod1