You can also check out your local Salvation Army, Good Will, or other thrift stores. You can usually find a choice of computers that are a few (to quite a few) years old for well below $200. As an example, Last year I picked up three HP PIII 866 desktops for $30 each. They even came with Windows 2K, 256K and CD writers. I know an PIII 866 processor and Win 2K doesn't sound too exciting, but it would be more than enough for AOL. You can't always find good deals, but the larger your city, the better chance you have.
Along those same lines, check out your local classifieds for people getting rid of their old systems, probably some good deals there too.