A vintage bike collection
Try Old Roads. I don't know how accurate this information is though.  

As 'user973810' suggested, try Sheldon Brown's Vintage bike pricing guide  as another reference.  Again, you may question the accuracy of the data.

 Vintage Trek has some original price lists and a lot of information on old trek bikes.  If you happened to be looking for information on treks, then check there as a guide for info about the bike.

If you had a schwinn, I'd recommend checking (maybe even emailing) Bicycle Heaven, a shop and museum, they have a massive collection of vintage schwinn and other bikes. Here's another good guide on finding prices for old schwinn bikes at Schwinn Cruisers. Their site has a lot of info about old schwinn models.

Check recent ebay auctions and see if there are any completed listings for that bike. Try using "completed items only" checkbox. It may still be hard to find results for some bikes. You may be able to compare prices versus their inventory.

Use something like  searchtempest  to search multiple craigslist sites to see if there were any postings for that style bike anywhere. These can vary somewhat wildly from actual value, both above and below, depending on the seller.