Easy,
Like everything, there's a product designed to fit every budget.
I'm not a big fan of getting cheap alarms. Those things do nothing more than make noise and un/lock doors for conveinence. These alarms will NOT prevent your car from being stolen or vandalized. Sorry for being negative, but I'm speaking from experience and hope to steer you in the right direction.
Before you even consider getting an alarm, check yourself first! What I mean is ..do you drive around the neighbourhood advertising that you have $$$ worth of stereo equipment in your trunk? Do you park outside because you're too lazy to opent he garage? Do you leave valuables visible and intice thieves.
Once you've got that in check, be prepared to spend a small fortune to protect something that is of greater value. Don't go out and spend $100 to protect something that is worth $20K. It just doesn't make sense.
I suggest you visit some of the websites of major alarm manufacurers (ie. Clifford, Direct Electronic, Viper etc...) Alarm prices vary depending on how flexable the system is in terms of adding bells and whistles to suit your needs.
Some basic features you should look for include;
-immobilzers
-backup pwr supply
-back up horn w/separte pwr
-keep in mind the more gadget the more it's gonna cost.
I'm a strong supporter of Pager alarms that tell you that the alarm was tripped. What good is it to you if your alarm is triggered but you don't know
A good security system is based on layering of features. Too bad those layers cost a bundle though. If you're serious about an alarm, be prepared to spend a good chunk of change. Grab a phone book and call a few shops up for quote. Hopw that helps.