There's no cast-iron formula
>85-90% through search engines.
This would be perceived as risky. From what I've saw (which ain't too much but the thread pops up often on here), 3-5 times earnings is a rough ballpark, so $3-$5K.
That's not to say that someone could monetize it much better than what you are already doing with Adsense, and value it higher.
Somewhat related threads [google.com] might yield some useful info.