Per month? Year? Decade? Century? Be clearer. Also it's not just how much it currently earns, but how long for. Valuing a site is different to a domain name.
And how long for? If a site has been earning consistently for 12 months (i.e. a proven earner), or more, for example then it's worth more than one that has just been earning that for 1-4 months (one-hit wonder, manipulated earnings, etc).