If it's a crack in the finish then the repair probably won't cost too much. If the actual top is cracked and it's not a very expense guitar, then I'm guessing it'll cost more to repair it than to buy a new one.
I once left my guitar on the floor of a friend's garage. Went inside for a bit, heard the garage door open, and went back to find my guitar lying behind the front tire of a car. Luckily, the tires ran over where the neck joined the body, and the only damage was a small crack and some scratches on the back.
dfi