In my experince Around 600-700 baht per day seems to be a fair rate for a good one, Check the tyres, pads, chains, wheel bearings and condition of oil etc and get them changed if need be, if hiring for awhile. Take own helmet also.
Hope this is some help
For those that are interested I do a buy back scheme on XR 250R's, but its only viable for periods OVER a month, because its will cost a minimum of 20,000. The advantages are : Late model bike, generally 2003 or newer,well serviced and registered.
Disadvantage against hiring is that you have to pay for the bike, with a guaranteed buy back.