I work it out to be about 65 yards of top soil per inch to cover half an acre. I didn't double check the math, but it sounds about right. Depending on the top soil density, it's probably about 1 ton per yard. That's a few dollars.
My lawn is about 12 years old. There were three rocks just at the surface that have bugged me the whole time. About two weeks ago, I went at 'em with my L35. Well, they were big enough that when I got them out and pinched between the bucket and the boom, I couldn't steer the tractor going forward up the slight (3% or so) grade to my rock pile. I had to drive it in reverse to get enough front tire on the ground to steer. Good times.
One other poster mentioned frost heave, and his experience mirrors mine. The grass and then the snow insulates the ground well enough that frost heave isn't a problem. It does happen on open ground, such as the driveway, but not the lawn. I'm in CT, so our winters are probably similar.
Good luck, and have fun.