Just to confirm a previous discussion. I just spent a few nights in BKK. 3000 LT no problem in Spankys, Angelwitch and Mandarin. I did not try any of the Rainbow bars, but from what I read in other forums it is more difficult to get this price there. LT was untill around 12:00-13:00 the next day (that's what I made sure before barfining).
I have been quoted 2500 ST at Rainbow 4. I laughed and did not even try to negotiate. The girl was still not barfined at 12
(I stayed there from around 10.30 to 12).
For the modus operandi, as usual: never talk price with the mamasan or the service girls, never look at the cards they show you, never ask "how much you want?", and of course be friendly!
For me it was just "I give you 3000 LT" "OK" (damned, should I have tried 2500?)
I also got two girls ST at 1500 each, but I knew them before. In fact they wanted to have fun in discos before going to my hotel, so it costed me a certain amount of drinks too!