In the movies when a car blows up or if it gets wrecked or something, Do they really destroy the cars or is it it a computer trick? If they really destroy cars, its a lot of $

The cars they blow up are not the cars they drove up in, but normally just a shell (plenty in the junk yard) painted to look good. I watched Myth Busters, and they had a good show on this. (The myth busters love blowing things up, and are former stunt men).

The cars that crash are reinforced with roll bars etc. And are driven by stunt men. Once it has been crashed, you can easily change the body and use it in the next show.

I don't know if this helps but I worked with a girl who dated a guy whom the people of the movie Fast and Furious wanted to use his car in one of the scenes. They paid him big money for it because it was one they crashed. (I don't know how accurate the story is. I only worked with her maybe a month or two.)