Greyhound is probably the cheapest. Its sure not fast, but will be a lot cheaper than anything else. Somehow it looks to be faster than Amtrak, despite stopping in a few dozen towns. I'm guessing the Amtrak route is a lot more miles and keep in mind that trains don't travel all that fast and of course Amtrak is always subject to delays. I've never had trouble with Greyhound, despite its reputation amongst some people.