Currently hotels in Bangkok vary from cheap to dirt cheap, depending on the area you're in. In the non-too-central areas a room for 400 baht can easily be found. If you're willing to stay in Bangkok for an entire month I suggest getting a furnished apartment though. For less than 10k you can find one for sure.
I am pretty budget-minded myself as I don't drink, eat Thai food daily and am not interested in women.
For me, even in BKK, 1500 baht a day would probably be enough, even when using the skytrain and subway.