The charge at Walker's is $30 per day, to maximum $199.50 per week, and then the price drops to $25 per day. I have never asked them about what the cutoff is for two weeks, or longer rates.
Their pricing basically is per day, but they will work with you as needed. For example, on my trip next week I will be arriving at my hotel about 7:00pm Wednesday, and leaving about 2:00 on Monday. They are charging me $135.00, which is 4.5 days (Thursday, Friday, Saturday, Sunday, and half og Monday).
Also, if you ask for one of the heavy duty scooters (300 to 400 pound capacity) the rate is higher.