Well, if you want to buy a site that makes $15/day - you're looking at $15x30 = $450/month - so you're looking to spend at least $5400 or up wards $6,000 for a site that makes that much, based on 12 months of income.
ForgottenCreature is definitely right. I'd never shell out that much money for a site and would rather build my own. That is unless you're willing to spend a good chunk of money and you think you can improve it's earnings to pull in a profit.
I would say if you found a site like that on Flippa it would usually go for around $2.5k to 6k. It really depends on a lot of factors. I have seen sites making absolutely no income go for 8 grand and sites making 1k a month sell for less than 1k.