That's very dependent on geography. A counter example: a train up or down the east coast (DC, Philadelphia, NYC, Boston) is often equal to if not greater than the cost of a flight. At least $100, usually a lot more. By comparison, buses are about $30, often less.

And that's to say nothing of extended networks - Amtrak is great if you live near an Amtrak station. A lot of people do not, and buses serve them well. Ish. For many places in the country, Amtrak is comparable to a slow plane, not a fast bus.