What about express bus service in Korea.
More frequent than Korail. Cheaper
Easily accessible from subway in Seoul. (Orange line line 3 & line 7, Express Bus Terminal Station) very clsoe to JW Marriott Hotel Seoul and Shinsegae Department Store
http://www.kobus.co.kr/web/eng/index.jsp
Seoul-Busan about 50 daily service (less than $30 one way)
200mile 4.5hr