According to Wikipedia, "The Republic of Korea has one of the highest defense budgets in the world (though second lowest in East Asia), regularly making the list of top ten." The 2005 number was about $21 billion. The country is able to produce a majority of the supplies and weaponry it uses, as "in 1990 South Korean industries provided about 70 percent of the weapons, ammunition, communications and other types of equipment, vehicles, clothing, and other supplies needed by the military."
Further, "the current administration has initiated a program of self-defense, whereby South Korea would be able to fully counter the North Korean threat with purely domestic means within the next two decades." I think that is a very important goal, both for South Korea and the US. First, achieving that goal would provide support for the South Korean economy. Second, with the unrest in South Korea regarding the presence of US forces in their country, I think that the US should withdraw all of its forces from the country eventually and let the South Koreans defend themselves.