The best way and cheapest way to reach COEX venue from Incheon airport is taking a Limousine bus(No.6103).
(Kimpo airport <--> Coex / there's no limousine bus running at this moment)
It costs about 14$ for one way. If your accommodation is nearby COEX, taking a limousine is the best way to save your time and budget.
Price comparison between taxi and limousine bus.
Taxi : approximately 60~70$ from the incheon airport to COEX
Limousine : 14$ from the incheon airport to COEX