HOBs would probably be your best "bang for the buck". A couple of AC110s or emperor 400s would work well. Or a mixture of both. I hear they all go on sale often and you can pick them up for ~$40 each. You should be able to find them at an lfs for ~$65 each or less.
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If you want to push the $200, you could go on ebay and pick up an FX5. I got the three of mine for ~$133 each, you see them commonly for ~$200 shipped. The FX5 will not come with media, just sponges, so you'll need to save ten bucks or so for plastic pot scrubbers from the dollar store.