I am going back several years, so things may be different now. The question came up at the time. I asked the boys then how much they get when a customer buys a drink for them. The answer was always the same in every bar - 20 baht. How much, if anything, they get now - I have no idea. Maybe some of you who patronize the bars can find out and let us know. Ask the boys, not the bar.
Even if the boy does get some sort of compensation if a customer buys him a drink, that does not alter my opinion that some of these bars are vastly overcharging for their drinks in the first place.