Thank you.
Now i thinking should I
(A) Stay at airport hotel , then next day go to city area hotel
(B) Take airport express to seoul station (20sgd per pax), then cab to seoul station hotel
Think most likely would go with option A in terms of cost saving.
I jiak ba bo sai bang. So I did a little research to answer your qn.
(A) Stay at airport hotel really depends on what kind of standards you want and expect. Staying in the airport transit hotel itself cost 170000W ($200SGD) for two for 12 hours. You need not step outside of the airport at all.
Other cheaper hotel options around the airport involved 5-10 mins shuttle bus bus services. Random example: Best Western Hotel (5 min walk to airport) cost 160000W/180000W per night (weekday/weekend).
You can of course look for further but cheaper guesthouses around the airport area but don't expect hotel standards.
But you still need to pay 3950W per pax to take the airport train the next morning.
Total cost: 170000W+3950+3950= 177900W
(B) The late night bus service cost 9000W per pax ($11 SGD) to seoul station. Once you reach seoul station, a cab ride to the close vicinity (cityhall/seoul station area) shouldn't cost more than 7000W (with 20% extra midnight charge). But it is also quite common for taxi uncles at midnight to refuse to run the meter and charge an exhorbitant fee to bring you to your destination. You can either bargain, or plainly refuse and wait for the next cab that is willing to run the meter. (I had problems getting a cab from dongdaemun at night after midnight shopping and I paid 12000W to go from dongdaemun to cityhall.)
But of course you must pay for your night stay at seoul's hotel and you must also check if they are willing to let you check-in past midnight.
Total Cost: 9000+9000+7000+Your one night stay hotel in Seoul
So all in all, I think both will cost around the same assuming if your seoul's hotel rate is cheaper than 150000W per night.
Personally I will chose 2nd so that I no need to keep changing hotels. It really depends on your comfort level.
edit:Read this blog for the cost of her taxi ride from incheon airport to hotel
http://thetravellingsquid.com/2014/02/02/arrive-at-seoul-incheon-airport-at-midnight/