Hi...
Ive owned my ultimate since 1990, they are brilliant ( once you get used to the handling )
Price wise/condition wise , any thing between £700 - 1000 .
top end if mint and comes with its accu-trax, even better if the FTW stem is included, the stem its self can be worth up to £300.
Since joining RB, ive come to learn about rust issue's with old steel framed yeti's..
Places too check :
Inside the seat tube, especially near to the bottom bracket.
if the paint is bubbling on the rear/bottom of the seat tube, it could be too late, and is rusting through...
The older frames with the bmx style seat clamp, can split around the clamp area.
Look out for frames with the sleeved top part of the seat tube. this has been repaired and is usually good done this way, If repaired this way, it will have a welded seat clamp as apposed to the bmx style separate clamps of the early frames.
I checked mine on learning the above, and have found no rust, and have since treated it.
The early californian frames had wider seat stays, and maybe some early durango frames, made narrower later, due to the use of narrow rims..
If you are lucky to find one, and its perfect. you will love it..
Good luck..
Martin