most compressor pedals worth buying will be in the $200 range or above, so expect to spend that much. I would recommend playing as many as you can before making a decision. A good compressor can do a lot for your sound if used correctly.
I settled on the EBS MultiComp and it is my favorite pedal i own. It has three different modes and i use them for enhancing my clean tone or limiting filter/wah peaks. It can be transparent or add a slight, warm, mid range boost to your tone. It also adds a really nice punch for slap.
Also check out these reviews and info: http://www.ovnilab.com/