Proving income for a website
I am interested in purchasing a new site this year. I want it to have an absolute minimum income of at least $20 USD per month.
With that in mind, suppose I find a site I like. What sort of proof would you consider adequate to confirm this income?
I operate two other sites. One generates a good income, the other is a pet project of mine that generates nothing. For my income earner, I could prove income by offering a paypal statement, or a google statement..but can't these be altered to appear as if they are generating a little more than they actually are?
So how do you prove income in a site you were interested in purchasing? what would help make you sleep at night?