Bar off, clothes on.

[cdnmatt]

Thinking about this, in your situation, I wouldn't really "negotiate". I'd come up with a number in my mind, and stick with it. For example, "hey man, want to take a quick break for an hour, go for a quick drink and maybe something to eat? Don't worry, no sex. I'll pay everything, plus tip you 500 baht to be nice. What do you think?"

If he says no, then just say "no problem", and leave. Always a decent chance when he sees you walking away he'll come running up and say, "ok, ok, I'll come". Besides, your deal would be more than fair any way.