what it's worth depends where you are, market saturation...deperation of the buyer/seller, condition etc. etc.
a desperate seller might ask for $300 in decent shape, but a desperate buyer that HAS TO HAVE A GT MACHINE LOLZ!!!!!111!!one one eleven!! gets to pay $1000 for a beat to hell machine.
where i am machines are pretty common, but dont come up for sale often, so a GT-complete in good shape goes went just recently for 1200. (he had to have it....)
a working GT97 went on "tradio" (call in radio show where we trade junk for beers or other junk items) went for 2 cases of beer... so $50???
so basically your guess is as good as mine.