From the information that I've been able to gather, the average selling price of a website monetized with adsense is 6x the monthly revenue (6-12 months proof)

Some other factors to consider:

Traffic - Keyword Ranking SEMRush - service for competitors research, shows Google organic and AdWords keywords for any site or domain | Home | Quantcast | Alexa the Web Information Company

Keyword Ranking - Google Analytics (Organic vs Paid?)

Intrinsic Value - value of content, web design, domain name etc