Erotic massage in Seoul, like everything else, is a tricky business for foreigners. Most locals will tell you that either won’t be served, or you’ll pay double the price. That’s one of the reasons why going alongside a local “Hyung” (older brother/guide) is recommended when you will be looking for a “massage parlor”. You know what happens next, you talk to the mama-san and follow her to one of the rooms. First, you have sex and then you get a massage, usually from a blind guy, like in China. Shitty thing is that you can’t pick a girl and you could be treated like garbage. It’s hard to put an address to these spots, because of the language barrier. Just try putting all these magic Korean alphabet signs into your Google maps. Good luck!
All I can say that erotic massage around Gangnam is far more expensive than anywhere else in Seoul. You can also look for the characteristic barbershop spinning pole signs. Those also provide a happy ending. I don’t know what’s up with Asian countries and their hair salons. I heard a funny story where the guy sat down for a handjob and got his eyes covered with a towel, barbershop style. 5 minutes into the action and he takes the towel off to the horrific sight of the owner (60+-year-old grandpa) wanking him off. The horror.