If you buy an after market alarm with remote start from a place like Frys, Best Buy, Hi-Fi Buys, Sound Advice, Tweeter or Circuit City they'll usually install it for free with purchase.
I should add that I have remote start added with my Viper alarm.
Cost me $500 for the alarm and remote start about 3 years ago.
But keep in mind that Viper alarms are expensive.
One thing that you should know also is that a lot of cars made from Ford or Chevy have a chip in the key that sends a signal to the car when it's in the ignition telling the car this is a legit key and ok to start. This was the case with my Tahoe and was a bit of an issue for the installers since they had to figure out how to bypass this when the remote start kicked on.
It ended up taking them like 3 days to get it working right and I still to this day have an issue where maybe one out of every 100 remote start ups it will fail to start, just turn over and sputter then cut off, when this happens it trips something in my cars computer telling it its trying to be started by an unauthorized method and activates a security measure that won't allow for it to start up until it's been locked and unlocked with the factory remote and then sits for at least 15 mins before trying to be started again.
Also just for your info, if you remote start a car and someone pops the hood, it'll cut the engine off as a safety measure and also if you hit the break while it's running via remote start it'll cut off the engine so that someone can't put it into gear while theres no key.