Onnuri English Ministry offers Korean classes every Saturday for foreigners living in Korea. Founded in 1999, OEM Korean classes are run by professional and experienced teachers from universities. One semester cost only ₩35,000, to which you will have to add the price of the textbooks.
For more information, please check their website.
To find more classes check out the 10 Magazine article about Korean classes in Korea, or see the Directory category here.