What are my options for transportation around the Great Plains (US)?

You could try your luck and look for a ride share at least for some parts of the trip, or see if someone wants to chip in for gas. I usually found some notices in the local hostels where backpackers ask or offer a lift via scenic route without being in a hurry, so that might work for you as well if you decide to hire a car and explore all the touristy spots along the way.

Another tip is to ask for a relocation at rental places - this means you drive a rental car from its drop off location to its next pick up location. This is usually much cheaper since they would otherwise have hire a tow truck or pay a driver, but you will have to meet a schedule and won't have too much time for sightseeing, unless you don't travel alone and can share the driving with another person.

Also try Craigslist, sometimes small companies offer cheap and somewhat regular lifts between 2 popular destinations in the local rideshare section. A guy in a 12-seater van took me from Las Vegas to Los Angeles, Door to Door for $25. Was not only cheaper than Greyhound, but also faster.

Megabus is hands down the cheapest buy for the value, granted you're on the Eastern side of the plains. Plan ahead, and anything is cheap. I went from Indianapolis to Denver for about $70 in 2007 because I bought 4 weeks ahead.

Always buy four weeks ahead, minimum.

As a native-Montanan and a traveler, I highly recommend renting a car when traveling in Montana. Bus service is expensive and irregular. What's more, many desirable destinations simply do not have bus service. Greyhound recently cancelled (see article linked below) many of their in-state routes. Megabus and other cheap bus companies DO NOT operate in the state. Amtrak's Empire Builder does still run through in the northern part of the state but most of the stops are in very small, remote towns without bus service; if you decide to de-train there you may not have the option of renting a vehicle. Also, the tracks often freeze for weeks at a time. If you're traveling in the winter expect delays. I have tried several times to ride the Empire Builder in the winter without success due to frozen tracks. Amtrak will refund your tickets in the event of a freeze.

On a personal note, much of Montana's allure lies in its wide-open spaces and is best experienced at a relaxed pace. Attempting to do it by bus won't allot you the time or freedom to truly appreciate what it has to offer. I encourage you to stop and linger in the small towns, let locals direct you to experiences you won't find in guidebooks or websites. Rent a car and explore!

Alas, if you must travel by bus, Rimrock Stages offers the most comprehensive routes. Here's their site:

http://www.rimrocktrailways.com/

You may find these sites useful:

• Empire Builder line description

• Wikitravel article on Montana

• Article on Rimrock Stages