I have been here few times: http://www.pullmanbangkokkingpower.com/ great pool and great hotel, it's a bit off the track so you need taxi of skytrain to get around. They are inexpensive so that isn't a problem. Something aroung 100usd/night.

Another place that i found great for location and cheap accomodation was this place: http://www.presidentpalacehotel.com/. 85usd/night or you can get the club-room which has "unlimited drinks" for something around 100usd/night if you are a heavy drinker. This hotel is not as plush as that Pullman but it has great location. 5min walk from skytrain and it's surrounded by lot's of shops etc. Pool isn't nearly as good as Pullman's.