we paid anywhere from 150B to 300B per day last summer, depending on location, quality of the scooter, and if there was any kind of insurance involved. i'd expect to pay around 1000B or less for a week, particularly if you have time to shop around. by the way, prices varied greatly in kanchanaburi, but after checking three or four places we finally found a decent bike at a price we liked.
let us know what you do find and wind up paying, just for a fram of reference. cheers.