Have you ever been to Seoul with airport bus? I just tired it last week to go  to Hongdae. Just simply buy the ticket at the counter (at the exit of the airport) and it takes 17,000 per person and around 50mins to get to Hongdae. The best one is you don't need to hold your carrier during the whole traffic since bus attendants or drivers help load and unload large suitcases.
Buses run on regular schedules from early morning to late evening, with night buses available for travelers arriving or departing during off-hours. So I think I will try to take bus next time also.