Pricing an Adsense site

[UBF]

We see adsense sites being sold by the hundreds (or thousands) every day but how do you know what a fair price is. Some of the factors that I think should be included in the calculation is:

- Income due to natural traffic visitors
- Backlinks and the quality of the sites linking to it
- Domain name - is it related to the main topic
- Domain age (the older the better)

But all of these are hard to put into an equation so if you were to price an adsense site what would be the fast way to do it?

Monthly income x5 or x10 or ??

I would appreciate your comments.