The main factor that most people look at is revenue. The multiple you get will depend on a lot of factors, but the big ones are the history (how long it's been earning), the traffic (how much and where it's from), and sustainability (is it a market that will be around for a while, or is it going to dry up soon).
I've sold several Adsense niche sites for 17x revenue, so they are definitely selling pretty well right now. All traffic was natural from the search engines and all income was passive, which increases the value since it's hands-off for the new owner.