Whoa, There! Apples to Apples...
The Mecum GNX (#468) had 5,000 miles, not 100,000.
Nonetheless, the Mecum car was quite poorly advertised and, as a result, was only bid to $54K (no sale, and rightly so) which was quite low. Bad spot on the block schedule also contributed.
A 100K mile GNX, assuming no rust issues (fenders, quarters, doors, etc), would probably be $35K to $45K. Low end needing some work, high end immaculate and well kept. This assumes original documentation and confirmed original turbo as well. Yes, it is possible to keep cars very nice through high miles, but it's not that common.
Case in point: I've seen some 20K mile cars look like they've been through a World War, and some 120K mile cars look like they just rolled out of the showroom (like my every day truck).
Only my opinion, which is worth exactly what you paid for it! :biggrin:
Good luck!