A few years ago, I had a guy ship me a bike on Greyhound from Florida. It was super cheap... around $50 for a 65cm complete touring bike. The guy that boxed the bike had never done it before and basically removed the front wheel and wrapped it in newsprint, cardboard, and packing tape. Consequently, the package was the size of coffin!

I think you have to drop-off and pick-up at the stations; and not everyone wants to visit the Greyhound station, but the prices can't be beat!