Good paint and no rust? For a '67-'68 probably a bit more than a '65-66, somewhere around 15k+...but that will vary a lot depending on whether you are talking factory v8, whether its been restored(and when it was restored) as well as what has been done to the engine, etc. I will be honest though, I would take a hard pass on a coupe if you care about resale value....even in the best shape with low original miles, they rarely bring more than 20k...in short though, running and driving but in need of at least a mild restoration 7-10k, needing no major metal work, with a paint job needing attention 10-15k, good running and driving condition with no rust and a good paint job 15-18k....show worthy paint job and a concours restoration(or low mileage original) 18-23k...anyone paying more than 23k for a coupe....there better be something really special about it, or it needs to be a fully built hot rod