You have a few options. You can get an alarm w/remote start for around that price $400 installed. It doubles as a security system, lock/unlock doors trunk all that. Or you can get a remote starter, which is much cheeper a lot easier to put in and you still use your stock remote as normal, the only thing this will do is start your car, so you will need to carry both remotes. Both systems can be installed without the ON Star intetrface. It will work perfect without it.
Word of advice...I would deff not buy or have Bstbuy install it...Go to a real Audio Sound place, one that only deals with car audio and alarms.