Before I left Korea, I shipped two boxes of my stuff at about 10 kg apiece (45 lbs total) for 60,000 won, or less than $60 U.S. All you need to do is buy a size 5 box from the Korean post office and ask for (literally) "slow boat" shipping. I sent my boxes mid-March, just a few days before the tsunami in Japan, so there was a little bit of panicking on my part. They gave me a tracking number, but I'm not smarter than a Korean Web site and never could find out anything about my packages. The estimate given when I sent them was up to two months.
The good news: They arrived in the last two weeks of April mostly intact! (Why one arrived a week later than the other when I posted them simultaneously, I can't imagine.) I recommend going crazy with the free packing tape your local post office provides because the boxes had taken a serious beating. Luckily, they mostly only held clothes and books.
Do take advantage of the unbelievably low shipping rates in Korea.