I can't answer for Pattaya but I always judge the adequacy of the tip on the body language and possibly verbal feedback of the boy. I've written elsewhere of a Bangkok massage boy who gratefully received 1,000 baht and wanted to know when I'd be back. In a commercial establishment 500 is probably a bit on the low side but there are certainly boys on Grindr who will ask for taxi money of 500 baht because "I not a money boy"