Working beyond the scope of a visa costs as follows:

Minimum fine:  1 million won
Over 3 months:  Up to 5 million
Over a year:  Up to 10 million
2 years:  15 million
3:  20 million (max)

Deportation and prohibition on re-entry are also possible.

The fines for arranging for such work start steeper but max out quickly (5 million to 20 million to start).

Source:  Same attachment as the overstay fines.
Immigration Control Act, Regulations, Attachment 7 (Standards for Determining the Amount of Fines), enacted February 29, 2012.
출입국관리법 시앵규칙 [ 별포 7] < 2012.2.29> 별표 개정 범칙금의 양정기준