It all depends on how STEADY the revenue is. If it's a NEW site generating some profit, I wouldn't pay more than 3 months worth of revenue. But if it's a site that has been consistently earning profit for over a year, I'd pay anywhere from 1 year to 3 years of the revenue as a purchase price. In some cases much more if it's an obvious growing market.
General rule of thumb to start a valuation is 12x monthly revenues. Add/Substract from the multiple as you see fit for other site factors (revenue consistency, topic, brandability, growth potential, costs, etc).