The first couple of times I barfined a girl I paid 200 baht. After that I was paying 300 baht.
I never went short time in LOS, only long time. I paid between 1200 baht and 1500 baht. I had thought that I would only give girls 1000 baht before i came here, but the girls I were with were very, very nice and i would have felt a bit cheap giving only 1000 baht for the whole night and well into the following afternoon.
I also offended one girl by paying her, but that is a whole different story.
I think that you should pay what you think she deserves. If she has turned cold, passive, uninterested etc the moment you get into bed then slip her 1000 baht and let her get on her way when she wants, if you have only recieved the very minimal level of service.
On the other hand, if she has treated you well above and beyond the call of duty, then show your appreciation!
Personally, I never discussed money with the girls that I took back to my hotel. I liked it when money was never discussed. I liked it when the girls trusted you to slip them an appropriate amount the following day.
For me, my only other previous knowledge of P4P, was in Amsterdam where 15minutes of clinical sex would set you back 35 GBP. Here, 1500 baht (21 GBP) can get you maybe 16-20 hours with a girl! It puts things into perspective when you are worrying about what to pay in LOS! Whatever you pay, it will probably be peanuts for you and if you treat them with respect then you will likely get great value for money!