I've tried it a couple times, and it kept turning out badly. My advice is, if you're going to pay an artist, you have to set deadlines right at the beginning, and crack whips over them. I'm a really laidback, understanding guy, and I think that's what kept screwing me over.
A big problem is, at our level, we can't afford the $50/hr most really pro, fast-turnaround artists are going to ask for. So we'll be stuck hiring talented amateurs for less money, and because they're amateurs, they're far more likely to start bailing out on you when the project is a few months old.
The problem I kept running into is the artist would start out strong at the beginning, producing to deadlines, but as months go on they'll lose interest and work slower and slower. New projects interest them more, and your payments start to be not as important as the fact they're bored or worn out on your project. Soon you'll be hearing "Don't send any more money until I catch up on the art I owe you, I swear I'm getting right on it!" And then they never quite catch up, and since you're withholding payment as asked, they start thinking, "Well, the money isn't rolling in anymore, so I'll devote 0.5% effort to his work, and 99.5% to this other work over here that's just started paying me too."
That being said, sites like iFreelance.com are a good place to post ads with your project and cost requirements. Just be ready to delete a lot of letters from companies that don't even bother reading your requirements (a lot of them seem to be from Korea, for some reason), and artists who look at your $500 firm budget, and demand $100 per background anyways.
Like the others said, too, consider the profit you stand to gain against your investment...talk to some other commercial game developers and find out how much they actually made back. It's fine to pay for a labour of love, but it's even better to have other people pay you for it.