I don't really know what the value would be. For comparison, I found a '66 Fairlane 2-door hardtop, bench seat, column-shift automatic, 390-2V car, in a similar condition, except no rust. Needed paint and interior and engine detailing, and a few other small things...got it running well enough to drive it the mile home...paid $1000. If cars aren't perfectly restored, I feel like it's really a buyer's market. It's worth whatever the least someone will let you have it for, since it's hard to find any two cars in the same condition for comparison. It can be useful to compare the asking price plus the estimated amount for the work/parts needed to complete it to restored cars, however...but on that car I couldn't help you.
It would be a classy ride, though. Sort of a more subdued alternative to the Mach 1 with most of the sporty attitude.
There's a 1969 GT registry online; maybe they could help.
I would think that one in fair condition; meaning it may have some rust but is in drivable condition with no major mechanical problems and is fairly complete that you would be looking at around $6k with a 390. If it is a "no. 2" car in very good shape and a typical local or regional show winner I'd say around $18k.