We shopped around for outsourced artists to make animated characters, backgrounds, etc when we were working on fuzzwich.com. The lowest prices we were finding for animations was ~$50 per second (for a simple, single character animation) and I think ~$100 for a static character. The quality often had issues (flickering, jumpy movement, etc) that would require in-house touch up. Those were from offshore agencies (China & India), so US is probably more expensive.
So for us, it wasn't worth it. We have an in-house artist, but wanted contractors on-hand so we could develop multiple campaigns in parallel. But at that price point, it was impossible to pass the cost off to a client and still make any money.
I mentioned we have an artist. Of the 4 founders, 1 is pure art (classically trained) and another is a designer/programmer. They've been invaluable, even now that we're doing no-art-required data/advertising stuff.
Extra art cycles are great for sales because you can go in with pitches that do all the creative work for prospective clients -- they just have to say yes. And if there's art downtime, they can also pick up a lot of the office management, PR, support, QA, etc work that typically cuts into development time.
Lots of people talk about getting art done via contractors, and it's probably affordable if your product isn't 90% art. The best process is to ask for samples. You can tell them what you are looking for (always send aesthetic examples) so the sample is relevant, but don't expect them to complete your task as a demo.
One warning: I'd recommend you pay extra for an artist who knows how to build art that works with your production environment, whether that's making webpage-friendly images, properly sized swfs, or whatever else. The last thing you want to be doing is having to format/adjust/whatever every file revision that arrives.