Generally speaking, a fairly new bike in good condition will sell for roughly half it's cost new. Subtract for a bike that is more than a few years old or in need of a little work. Add a little if if it like new and hardly ridden. My son bought a K2 Zed 4.0 two summers ago for $300. This bike was hardly ridden and in like new condition. I hope this helps.
As far as age, you might check here:
http://www.bikepedia.com/QUICKBIKE/B... 4.0&Type=bike
You can click on various year to get an idea of age