In an alarm I would want at least 2 shock sensors, a glass break sensor, keyless entry, trunk pop ( I have a sedan), and the alarm system itself. VIPER makes one of the best systems, base model should be about 250 installed. This will be 2 shock sensors, 2 3-button remotes and the system, for 50 bucks you can add the glass break, 35 more you can add the keyless, 35 more the trunk pop. Make sure that the system can handle the auxiliary features. More advanced things from there are 60 bucks for a voice module, 90 for a programmable voice module, 60 bucks for a proximity sensor. These are all installed prices, and you don't have to have it done all at once, you can always just do the base, and then add on the other parts later. As you move up in the price range you get more features, more auxiliary functions (valet, led warning signals, more shock sensors, a pain inducer (screach like unbelievable), if you have an ATX you can get remote start, and then you can get 2-way alarms, where your Key fob, will communicate with the car, and the car with you, it will "page" you if the alarm is set off even if you are out of hearing range of the alarm, it will also tell you when your doors are locked or unlocked, trunk open, windows down or up, armed or disarmed, started or not, if the alarm has been set off while you were away, how the alarm was set off. The 791VX Viper system runs around 650, with most all of these features.
I purchased a Viper not too long ago, don't remember what model but it ended up costing me $315.00. The one thing I would recommend, especially if you have a decent stereo system is the extra siren. I had my stuff jacked and when I was talking to a Tech guy at a shop near me he mentioned it will cause someone to lose their hearing if theyre in the car for 2 min or more. The siren is mounted under the dash near your stereo and it's the highest pitched thing you've ever heard. Thats all I cared about, if you're gonna steal my stuff, I hope it's worth you're hearing. Another one I would reccomend as long as you don't live near Mexico is the LoJack. tracks your car when it's stolen. I've had 2 friends get their car back within half a day, but the one friend who lives near the border didn't get it back cause it was stolen while he was asleep and was long gone in Mexico by the time he woke up. Think thats around $800.