The Thai Spa Gangnam is located in the world famous now neighborhood of Seoul. The location is just a few minutes walking from the subway station Sinnonhyeon. Prices are not cheap but still OK by Korean standards.
Some of the rates off their web site are:
Foot massage at 70000 won for 1 hour Asian core body massage at 80000 won for 1 hour Anti-stress massage with head massage and herbal treatment at 150000 won for 100 minutes.
There are many hotels in the area and some of the ones near this location are: Best Western Premier Gangnam, Novotel Seoul Ambassador Gangnam, Samjung Hotel, Ritz Carlton, California Hotel Yeoksam, Gangnam Serviced Residences, Yeoksam Human Starville and Noblesse Hotel.