There are a lot of variables. I'll give you a quick run down of my personal opinion.
I would say that I could spend up to $3k on a normal old G body that is clean, unmolested and unrestored.
..........................................$8k on a more rare car like an aero coupe, 2+2 or GN (I don't care for the 442, sorry) that is clean, unmolested and unrestored.
..........................................$10k on a normal old G body that is frame off perfect show winning.
..........................................$30k for a more rare car of the same condition.
Some period correct and tasteful modifications can be made. I wouldn't change the price over them.
But if I see 26's, Facebook painted on the side of the car, the wrong seats, dash, console (no matter how well they fit) a system in the trunk, or anything else I find gaudy/tacky/doesn't belong, I'm running far away.