Right now the least expensive certified ADSB-out is the King KT-74 at around $2400. If you have a KT-76 now, then it is a slide in replacement. But you need a WAAS signal from a certified WAAS receiver like a 530 or 430 as the KT-74 does not have an internal WAAS receiver. Also the gray code from the altitude encoder needs to be brought into the KT-74 at install time. The KT-74 uses the existing transponder antenna and cabling. If you would be replacing an old NARCO or Garmin 327 transponder, then the install is a bit more costly, but not much. Total all in should be around $3K if you have a certified WAAS receiver to get a signal from.
I just installed a Garmin 335 for a friend. The 335 kit came with a solid state altitude encoder GTE170 from Garmin that was stuck on the back of the install rack. The encoder is .75 by .5 inches in size with just 3 wires going directly to the 335. The 335 can use the old transponder antenna and cabling. All I had to do was transfer over the 12 volt + and_ wires from the old KT-76 and fit the rack which is the same size as teh KT-76s. The 335 has its own internal WAAS gps receiver. We then test flew the plane in approach control airspace for 30 minutes to do teh ADSB test required for a certified install. Within an hour the FAA responded with a clean report with no discrepancies so the owner got his $500 rebate. The 335 kit from Chief's Aircraft was $3400. We threw away the old encoder and all the wiring as the new tiny encoder worked like a charm. The owner then bought a Merlin from IFly for his ADSB IN to play on his IPad.