depends on what you want
they can range anywhere from $100 (not including installation), You can give mongoose a call (not sure on how much they charge for their alarms), or JB hifi, wow sight sound, autobarn, supercheap.
you want one that atleast conforms to insurance standards, so i'd be speaking to your insurance company to see what they require to be covered, then go to the shops and tell them what your insurance company require.