The "BlueStar" onstar replacement is like $300 if I recall correctly.
If you have a tape deck in your radio, they make an adapter for that, but they are common knowledge so I'm guessing your unit doesn't have a tape deck.
I believe they have FM transmitters fairly cheap, which you plug your MP3 player /etc. into and tune your radio to the transmitter's frequency. I've heard the audio quality is hit or miss this way, but it's a simple solution.
Lastly, you could potentially open up your radio and install an aux jack in there. I've done this with a couple of radios. Basically, you find where the left and right audio come from the CD player (don't forget ground!) and tap into them for your jack. Then you put a blank CD into the CD player, and the car will think it's playing a CD, but instead play whatever you plug into the jack. Radio Shack has the jack port that you can install, only costs a few dollars plus some solder and wiring.
