look on ebay for some viper alarms. I don't know their prices off the top of my head, but i think you can get the 791vx for 200, which is a steal (very very good alarm...but it's out of your budget)
x2 It's amazing that the safety and value of your car and equipment is only worth $100 to you. Really though installation is the biggest part of an alarm and the difference between an alarm and a security system.
I got my Avital 2100? (not quite sure of the model number) installed at a little car audio shop near my house for $100 new. Its wired so if armed and someone opens the door then it will go off.