Ultima isn't expensive... I just sold V and VI for just over £40 for the pair, new in box.
I think the Ultimas value are really the Apple II versions. I've seen some of the C64 versions fetch a decent amount also...
It's tricky, because the Amiga doesn't seem (to me) to be in that "collectible" age/range yet.. It's not one of the first like the Apple's (No, don't go into the Altair or the SOL's here.. ;-) .
The things that are really expensive for the Amiga are the things that are rare and yet people still want to use (68060 cards, video cards, etc)..
When I see sealed copies of Mindwalker actually selling for hundreds, then we'll have reached collectible status..
IMHO
desiv
(p.s. Have you tried to buy a sealed (well, in it's plastic baggie) copy of Akalabeth for the Apple II??? ;-)