A few things (besides revenue) off the top of my head that could effect value: Design Required maintenance Uniqueness Domain Name Where the revenue/traffic comes from Basically anything good or bad about the site can positively or negatively change its value.
I'd say that the most important factors (on this forum) are: #1. AVG. Revenue that you can PROVE is real #2. Traffic, that you can prove with decent stats #3. Turnkey or Original design #4. Original content and # of pages indexed (co-op wieght/link vault) #5. Domain name & PR, also AGE of domain/site #6. Required maintainance to keep the site going. I'd say that these are the most common metrics. $1-10 /day... so a mean of $5, if you can prove it you'll probably get $1500 If you have exceptional traffic/original content/high PR, you'll get more.