I saw a recent ADSB transponder install in a Piper Commanche (Bendix King) and it cost $5k.
Straight slot in for either KT74 (Trig TT31 with King written on the front) or a Trig TT31 same rack is doable for around that price, just need to run RS232 from GPS to Transponder rack - but you need to already have a suitable GPS.
Exact details are hard to decipher but it is generally accepted that a TSO C145/146 WAAS GPS will be required to provide a suitable GPS data source for ADSB.
So it depends on what you have fitted already, an Garmin 430/530 can be upgraded to W spec, other stuff your are looking at new units.