I used to stay in Watana mansion. Rooms are rather decent. If I remember correctly, the charges are about 500 bahts then. Perhaps it may inflate up to 600 baht. Ok facilities. Nothing to shout about. There are plenty of Caucasians hanging around in the hotel.
Plus point: Rather cheap, Near to Poisedon, Accessible via MRT (walking distance) and to MPs along Ratchadak stretch.
Minus point: A bit run down facilities, cheap location.
I've not been to BKK for almost 3 years. Hence I'm not sure if there are any changes since then. Perhaps any regular senior samsters are advised.