Move. But seriously, there are so many different types of car alarms, and labor prices vary a lot around the country it's real hard to say. I would think you could get it done for $200-300 dollars.
A car alarm won't stop your vehicle getting broken into, and smart thieves will throw stones at cars with alarms to create false alarms until you either shut it off, or start ignoring it. Move, get a garage, have a motion-sensitive floodlight, or sit outside with a baseball bat / shotgun / hunting rifle until they come past! As the other person said, it varies a lot with quality of the alarm and how much labour you get charged, but expect to pay a good few hundred $ for something worthwhile. Make sure you have nothing on display in the car which they might want to get at!
