I paid towards the top end of the $400 range (about $470 I think). I went through Action Alarms (good people, also a SAUWA consolidated workshop), came out to my house the same day, fixed my central locking (wasn't working properly), installed an immobiliser aswell as a kick-ass alarm system with built in turbo timer.
Give Action Alarms a call, I cannot reccomend them more.
Or if you're around Osborne Park Westside Car Audio do a good job also and are very friendly.