I always wonder how these sources that claim to know muscle car values get their numbers. There are so many variables in cars that it is difficult to impossible to know beyond an educated guess what a car would really sell for IMHO.
Some of those variables include: #'s, unrestored or survivor, color, 4 speed or auto, certain options like shaker on an E body, how many beers the bidder with more money than sense drink at the auction, if there was shill bidding at the auction, does the bidder at the auction have a trophy girlfriend with him that he is trying to impress, how much have certain cars been selling in private unadvertised sales, etc.
Seems like you'd need a pretty complicated computer program to analyze all that data and spit out a number. Even then, that number might not be much more accurate than next weeks weather forecast!
Many of these places that talk about collector car prices have a stake in selling cars and this is just part of their hype to drive values up and make themselves more money. Investment grade this, flipper!