Bars in Korea can get pretty damn expensive to drink in. The cheapest and best drinking I've come to know in this country is sitting outside of any convinence store. Family Mart, 7-11, GS 25's doesn't matter. When you run out go back inside for another. Most will have a table and chairs outside just for you to sit and drink.
This picture is courtesy of Dennis in the Freedom Riders MC on a ride to Chuncheon.