i have never been to one that it's around Pratunam area. So far, i have only tried those spa located inside hotel but those are really quite expensive, easily at least 2500 BHT. I usually go for the cheapo thai massage located along siam square...only 450 BHT for 2 solid hours!
You may want to try Samsara spa. The pics in my bangkok map look inviting....it's quite near to Chatuchak market, about 2 BTS trains station b4 Mo Chit.