I find it's highly dependent on the route. I can get from my hometown (Madison, WI) to Detroit by plane for a little over $600. Amtrak stops at a small town a little north of Madison (about a 20min drive for me), and can get me to Detroit for $150. Even a private room is a bit cheaper than a plane.

Train does take 16 hours, but it's 16 relatively pleasant hours where I can stretch out without being cramped. It would also be significantly faster if we invested in rail properly. Hell, maybe not even high speed rail; that route could probably be a lot faster if we just made Amtrak suck less.