Here's a website that will give you an idea of what hotel costs are, as well as what you will get for your money: http://www.hoteltravel.com/vietnam/index.htm
I live in Asia and have traveled to Vietnam twice this year, everything else will be quite inexpensive. Three days in Hanoi, with a family of three, seeing many sights, taking taxis, meals, a few cheap souvenirs and we spent around $120.00 US between the 3 of us for 3 days, not inc. hotel. If you're going to be hitting the bars and nighlife, that could easily double the money you spend. Of course, it never hurts to bring a little extra cash and/or credit cards just in case. But overall, Vietnam is very inexpensive, especially for an American. Have a good trip.