i paid $550 for mine, including central locking and fitting.
go to a reputable stereo shop ask them prices and go from there, also ask them if they can disable the door key locks so if some arse clowns tries to punch the lock it aint gonna do shit except push the lock into the bottom of the door cavity.
mongoose are good plus insurence companies recommend them on there list. as for central locking from my experience aftermarket ones tend to lock like 95% of the time so just check before u walk away from the car
lol i like hte mongooose m60 =] its upgradeable the base product is okay but if you wanna go way gizmo overkill you can get Gps tracking and all this other junk the alarm even msgs your car if say your alarm goes off i dont know if it tells you about the windows getting broken cuz i know you can get the glass sensor . oh and if you have that sms upgrade thing you can sms your car to unlock your doors lol incase your dopey like me and left your keys in the ignition. lolz (upgrades obviously cost more) and sorry if none of what i just made sense lolz i cant think of any other way to say it