I've been hospitalized in Bumrungrad three times now for "acute gastroenteritis", once for three days, twice for one day each. In all three cases, the final bill was under US$1000. In the US, hospitalization starts at around $5000/day.
I had to hit their ER one other time, treat-and-release. I don't remember the bill, but it wasn't enough to get excited about.
I have had one bad experience with Bumrungrad's billing department. They tried to tell the nurse to keep me in the hospital for another day while they sorted out something on the billing. The nurse refused, saying that all the medical work is DONE and the patient is perfectly willing to hand you his credit card.
Bumrungrad is at least as good as any mainstream US hospital. Their ER is on par with the best I've seen in the US (and you do NOT want to spend enough time in ER to know how to spot a good one).
Some Thais don't like Bumrungrad, as their fees and some of their medication decisions are based on the assumption that the patient is foreign and thus has more money. I had to run a friend in one night, for an acute allergy attack. She refused to let me take her to Bumrungrad, and insisted we go to Police General Hospital instead (a few more blocks away, no big deal). I assume she had some kind of medical insurance, because the final bill was the equivalent of a few dollars US.