If you know a little about your car's wiring and alarm components or are willing to take some time to learn about them (SPDT relays, diodes, etc), then get one on eBay. I got a Viper 671xv on eBay for about $120 and installed it myself in about 4 hours a couple weeks ago. And I taught myself everything a couple days before I installed it lol. www.the12volt.com was extremely helpful.

BTW, go for buy it now's since eBay likes to randomly delete Viper alarm auctions.