The highspeed rail (based on TGV technology) from Seoul to Busan takes about 2:45 with 2-4 trains an hour. Ticket cost about 45,000won ($44) one-way.

The train between ICN and Seoul Station is being built; right now, there's a coach that runs every 15-20 min, takes an hour and 13,000 won ($13).

And there are a few flights a day ICN-PUS. The KE flights leaves at 8:05pm. That probably give you too much time for a transit from the US, but it saves you from the coach transfer and/or the train ride.