I didn't know that budget charged only by the day, as the standard is to give you an set number of days based on the distance you'll travel and an amount of miles that covers the distance between your two points, plus some extra for the inevitable extra driving that you'll need.
I am relieved to see, however, that while they charge by the day, they do have a minimum number of days that they will charge for. (In this case its 11 days) I could see people trying to drive way too far and too fast to shave a day or two off their rental cost otherwise.