The traditional Thai massage and oil massage shops in Bangkok are world famous for their special technics and pretty ladies. You can enjoy a nice massage usually from the early noon to the late night.
The best time to visit is between 18.00 and 20.00. Most of the girls will be available at this time and you can have the best choice.
It is not expensive but you should expect around 300 Baht for one hour Thai body massage. A tip is not necessary but expected. If you were really happy with your massage an extra 100-200 Baht is enough to reward the lady.