For many routes, it is dirt cheap, but like anything, you get what you pay for. I take it semi-regularly from Walla Walla to Seattle (approx 5 hours by vehicle) and it is about $50. If you plan ahead, there are flights (45 minutes) for $60, but they can run as high as $200. By and large the Greyhound consumer is of an income bracket where that $50 is a significant expense, and an airline ticket would be a serious stretch. Also, Greyhound appears to have a deal with the prison system, as released convicts from the state prison are regular passengers. Frankly it isn't any worse than an airplane and you get regular stops to stretch your legs and get a cup of coffee. All the busses have wi-fi and some also have power outlets.

To your point -- for really long distances, air travel is likely the way to go, but for regional transportation it is a reasonable alternative. A tank of gas in my Ford F-150 is as much as $90, making a bus ticket or a flight cost effective within the region.