@Dick
$4,100 for a Trig 31, which looks like a really good unit and meets all of the requirements
Trig TT31 Mode S Transponder
The Trig 31 slides into the existing mounting tray.
The Avionics LAME estimates less than $1,000 to fit the unit, including the Engineering Order and testing.
My old Narco 150 is on it's last legs and has been causing problems with ATC and needs replacement anyway.
To upgrade to a new Mode C transponder would cost ~$4,000, so spending $5,000 on an ADSB unit makes sense.