I think the Ultimas value are really the Apple II versions. I've seen some of the C64 versions fetch a decent amount also...
Ultima: Escape from Mt. Drash for the VIC 20 is probably the most valuable one. I've still got my original cassette, but sadly the box and manual are long gone.
http://en.wikipedia.org/wiki/Ultima:_Escape_from_Mt._Drash
