I think the answer for current bikes is that it depends.

Some bikes are sold frequently, and if you watch several of the sales, you can get a pretty good ball park on its average value on ebay and in your CL area.

Rarer bikes are more volatile on the market because there are fewer data points. When you're dealing with truly top end stuff (I don't mean an 80s Colnago, I mean a Confente), a lot of the time the value is just whatever you come to agreement with.

What you can't do is use irrelevant data points...like draw conclusions because of tubing. You also CAN'T confuse craigslist and ebay values...the venues are very different. Another error I see people make frequently is using what they'd pay, or what they have paid, as relevant in offering advice on average value in a TYPICAL sale (meaning selling it within 2 weeks, not having to scour ads and dumpsters to find one at that price).

If you're talking about predicting value 20 years from now, that gets more questionable. Generally truly top end stuff will go up, but not always. There are trends (see balloon, Schwinns).