Establishing the "value" of an antique car is a tricky proposition. One way, of course, is to monitor current sales prices from key selling venues such as eBay, Hemmings,and other LBC mazazines, etc.
Also, value may be linked to the actual, direct cost of restoring a LBC. To restore a rust bucket, non-running GT6 to "good", driving condtion costs roughly $5,000-$7,000, and that's if you do all the work yourself (ugh). In this light, $7,000-$8000 is a <u>very</u> good price for a nice example.
I'd happily pay $10,000 for the right GT6, maybe more for a concours car. Everyone's situation is different. For example, I can't do body work/paint, the most expensive part of any restoration. Thus, I'll pay more for a no-rust, straight, painted car.
These factors tend to increase the purchase price of antiques over time. Who knows, maybe a $20,000 GT6 is not so far into the future?