The cheapest new radio modules I know of are from Crevier or Pacific BMW, $420ish. Every once in a while you will find one on ebay used. There is a 2005 radio module ebay right now in fact. It is already up to $400 for a used one. I am not sure the cheapest place to buy the Sirius kit but I would check those two places first.

Depending on how you are about sound quality, you can go with a good RF modulator for Siruis and skip buying the radio module. I wouldn't do it personally since you are doing a real disservice to that digital sat signal coming in, but it is an option to consider. If you hated it you could always buy a module later or catch a used one on ebay.