On the weekend we had lunch at this small Mexican spot located in Itaewon. If you get off at the Itaewon subway station, take the Itaewon Market exit. The moment you exit the station, behind you should be Burger King. Walk towards BK, turn right and walk down the street past Gecko's Bar & Grill (you know I'll be visiting there in the future). About 100 meters later you should see Taco Amigo (Thanks ZenKimchi!) on the right. I had a beef taco (with cheese, sour cream, olives) with chips and salsa and refried beans. Add a small can of Coke and my meal was a reasonable 7000w (7usd).
Don't mind my snot rag in the upper left corner ruin your appetite! ;-)