What might be causing low compression?

I have a 2005 chevy trailblazer that we bought a little over a year ago. We have had to fix misfire in cylinder 4 about 4 times in a year. Recently car stared idling rough, overheating, no pickup, drives terribly slow- had it towed to shop. They said there was a misfire in cylinder 2 and 4. Now they say compression in each cylinder is about 80 and supposed to be (250)if I heard correctly. They say we need a new engine. Do we really? What could cause this low compression in each cylinder and is there some other fix we can do that won't cost thousands? Thanks!