As far as I know it is MUCH cheaper to buy a Japan-Korea R/T in Japan (as low as $200) than a Korea-Japan R/T in Korea ($5-700 from most Korean travel agents).

If you must do a ICN-Japan segment, try to get it included in your longhaul ticket, even as a sidetrip. KE charges about $200 extra for ICN-NRT-ICN in Y as part of an Europe-Asia ticket, and KL can add a stopover in ICN on an Europe-NRT ticket at a low cost as well; I suppose other carriers, especially with a partner flying this route, can do the same.