How do you find out the value of older bikes?

I found a Giant MTB for sale on Craigslist - an '03 - for $150. It looks in pretty good condition in the photo and this is in my neighborhood, so I can easily go look at it, and the guy is asking "OBO" so I'd like to go in with a firm ceiling amount. Is there a Kelly Blue Book for bikes?

Roxy

http://sandiego.craigslist.org/csd/bik/2242330467.html

Great idea, though. But for bikes, I think it's truly whatever the buyer and seller agree to.