After calling immigration, and being told that there were no fines (definitely wrong, but that's the person you want to speak to if you have violated the law, ha), I looked them up and here they are for you. Although current as of August 19th 2014, please note these can be changed easily by immigration.
Overstay period Fine (KRW)Less than 3 months 100,000 - 500,000
Less than 1 year 500,000 - 2 million
Less than 2 years 2 million - 4 million
2 years or more 4 million - 20 million
It appears the thinking was that every three months is worth about 500,000 won, and that each year is worth about 2 million won, although there is substantial discretion for officers, particularly in the last, largest category. Like any order, a fine is subject to a court review if you are willing to expend the money and time.
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