Under a $100 bucks is pretty tough. What about one of those Alesis NanoCompressor thingies? You also may be able to find an old DBX 116, 117, 118, or 119 for that. I think those are the model numbers, at least a couple of those will be compressors!
But in the sub $100 range you might be better off going with plugins. I think the stock plugins that came with Digital Performer sound better than any of the sub $100 compressors I've heard. If you want a trashy sound, or an effect type compressor, then a sub $100 unit could work out really well. But if you are loking for something that works cleanly and unobtrusively, then you might want to go with plugins.