I look at several places for "selling" prices (not value).
1. eBay
2. PCA's website
3. Pelican Parts (sometimes there are cars listed)
4. Rennlist
5. Autotrader/Classic car Trader
Just from these few lists you can draw up a pretty accurate price on what you're looking for.
But like everything else in life, it's worth what someone is willing to pay for it.
EX: 2 928 GTS's were listed on Ebay. One was $25K, the other $70K. Both were black, w/5-speed, low miles (less than 30k).
Difference: one was a 1993, one was a 1995.
Question: would you spend another $45K, just because it was the last year the 928 was produced? Not me personally, because I'd take the other $45K and buy a '60s Hemi Mopar for restoration (assuming I had boat loads of money just sitting around.)
Short of that, get the (reliable) inspection done before you buy......
regards---rhjames