Here are some sites that sell used equipment - and in particular KT76s. Looks like the price range is from $440 to $900:
http://www.wingsonline.com/avionics.html
http://www.avionics-wa.com/usedequipment.htm
http://www.avionix.com/xpndr.html
The first link is where I saw a KT76 for $440 (posted 6/1/02) - but you have to scan down quite a ways to see it.
Also, if your avionics shops is an AEA member, they have accessz to an online database of used equipment for sale - and that's maybe the best place to look.