Still taxi is very expensive. Even after 10PM, there are some schedules available.
http://www.airport.kr/airport/traffi...ist.iia?flag=E
http://www.airport.kr/iiacms/pageWor...=1285334449919
In addition to this, KE and OZ has a couple of 11pm-midnight arrivals, and they arrange buses to downtown. Oh, that is also included in the second link.