Yeah, your own garge or driveway is definitly the cheapest place around. Failing that, a friend's garage.

If neither of those options work for you, don't go cheap. Don't go outrageously expensive either. It's a good amount of work to tear open your whole car, run power down one side, signal down the other, mount a head unit and mount an amp. Most people who do it cheaply aren't going to put the time and effort into doing it right, and they're going to make up for the install price on the price for the components. Remember, the price for the install is the total price minus the cheapest price online for the components, not minus the list price.

Matthew