£2500-£2800 plus VAT is about right for a Garmin GTX330. The unit lists at about £2000, and the cheapest installation (assuming a readily available prewired bay in which a transponder used to live) is about £300.
This is for either a G-reg or an N-reg but on a N-reg it costs more because an IA has to sign it off; alternatively you can go to a firm which has an in-house IA but they will charge you more to start with
If you need a DER signoff that's another £500 at least.