We have the BFGoodrich Sky497 Skywatch system, which is a TAS (Traffic Advisory System), not a true TCAS. It only sees other aircraft with a working transponder, but it does a good job. We see lots of stuff that we would never see at all if we didn't have it installed. It doesn't do resolution advisories or anything fancy, it just has a display which fits into a standard hole in the panel, and a voice interface which announces "Traffic, traffic" when it thinks there may be a conflict. I have no idea of the cost, but I do think it's worth whatever it costs.