Cheapest? Probably some no-name brand on Ebay. Other than that, if you want name brand....I would look into Audiovox or Omega. Anything else will most likely be more. But with most things, you get what you pay for.
you can go for the Clifford 3305X 2-Way Security Alarm. goes around $135 . its probably the cheapest 2 way DEI brand security/alarm system. not sure how much lighter your wallet is gona be if you have it installed by local installers, the installation guide is pretty straight forward. if cutting cost is the goal, you can simply DIY.