Average price I paid is around $200, shipping is always the killer so if you can find one locally, then better. With shipping I paid $200 (truck) and $275 (Supra) and ...
The kit in our TR3 came new in the box from a failed project and I paid $800 for the kit, hydraulic t/o and a nice Supra tranny. I found it locally so no shipping either.
These trannies are used in several different conversion, not just TRs so are not all that easy to find, but they're out there. Rebuilding is very expensive so look for a good one.
If you opt out of the hydraulic t/o bearing (cost around $300 just for the t/o w/o ss lines), and go for the master/slave setup, you'll save some $$ but IMO the hydraulic is a really nice way to go. The hydraulic t/o hooks up directly to the master and there is no clutch arm or push rod to fool with, ever.