If you want info on domestic flights by Korean Air, you will have to go to the Korean region website in English. If I recall correctly, you might have to sign up as a member and log-in to view the fares. http://www.koreanair.com/ Ensure you select 'Korea' under region, then toggle to English site.
Other airlines serving Seoul-Jeju include Asiana Airlines, Eastar Jet, Jeju Air, Jin Air and T'way Airlines. Jeju-Busan will be Korean Air, Asiana Airlines, Jeju Air and Air Busan.
Price for Korean Air and Asiana Airlines will be somewhere around 80 000 KRW for Seoul-Jeju route. The other airlines are "budget airlines" and will cost slightly cheaper maybe about 60,000 won. The earlier you book, the better discounts you can get for these "budget airlines".
For Jeju-Busan route, similarly price varies with discounts. Via Air Busan or Jeju Air, you can get flights for as low as 40,000 - 50, 000 won, or up to at least 70,000 won. Asiana and Korean Airlines will probably cost about 80,000 won or so. All including taxes.
T'way and Jin Air, if I recall, will require you to book via their Korean website.
Edited: 8:00 pm, March 04, 2012