Frommer's lists exact prices in the local currency. THowever, rates fluctuate, so before departing consult a currency exchange website such as www.oanda.com/convert/classic or www.xe.com/ucc to check up-to-the-minute rates.
As most parts of Montana is connected to the broadband backbone, you'll find credit cards are accepted at most lodgings and restaurants, and many attractions. ATMs are also fairly common in towns of all sizes, and even in the national parks. But these states also have numerous back roads that lead to out-of-the-way places with establishments that do not accept credit cards; I recommend you carry at least some cash on hand (about $200 is a good amount) while traveling in either state.
While the area is still relatively inexpensive in terms of the cost of hotel rooms and lodging, there are also many luxury establishments in places like Whitefish and Jackson Hole. You'll find plenty of $50 double motel rooms, but lodgings in Yellowstone gateway towns and ski resorts average about $150 in peak season for a standard room. Nevertheless, there are vacations for almost any budget, from camping and cooking out in the national parks to the pricey rooms at Four Seasons Jackson Hole.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.