You could probably get some scraggly flash animations for like $50 a piece, but if you want something nicer then it's likely to reach into hundreds or thousands, depending on the quality and how much you want.
I did a little research and a lot of "freelance animators" seem to charge hourly and are likely on the more affordable end; $50-$100 per hour seems the most common rate, keeping in mind that I've also read that with animation "you must expect to give hours for seconds", so you'll be dishing out quite a bit. But like I said earlier, for more amature work you could get much cheaper but expect to get what you pay for (Firehearts, lol)
Basically, this is a pretty unrealistic goal unless you have A LOT of cash to throw around. Your best bet would be to either settle with a very mediocre flash artist or get somebody to draw the frames (or you could, I don't know if you're an artist) and slap them together in Windows Movie Maker yourself.
Animation is a long and painful process, people aren't going to do it just for pocket change!
EDIT: I'm sort of talking out of my arse here but I'm probably correct about it not being very affordable for a teenager (of which I assume you are one; you come across that way, apologies if I'm wrong), though it might be worth doing some research of your own, too!