I was in hospital in Koh Samui recently, and treatment was expected to cost app 20,000 baht. I had to pay 30,000 baht up front AND leave my passport with them as deposit in case the treatment was more, and I did a runner. I thought that it was illegal for anyone to ask someone to leave their passport as collateral for anything, and they already had 30,000 baht cash, but all they would say was that it was "hospital policy", and they made it quite clear that I would not get treatment unless I complied.