Your least expensive option would be KAL limosine bus. The buses (sky blue color) runn every 15-30min from/to the airport. It cost 12,000 KRW and goes around hotels. After you clear the immigration and custom, look for ground trasnportation/Bus sign; there is an area where all of the buses depart.
http://www.airport.or.kr/Eng/transportation/bus.jsp
You purchase the ticket from an agent and make sure to get on to the one marked City Hall/Westin Chosen/(Seoul) Plaza Hotel/Lotte Hotel line.
BTW, bus driver/agent will help you with luggages.
On your return, you can purchase the ticket at the hotels' front desk (you can room charge for extra pts).
If you need an mobile phone, rent it at the airport; the rates were 3,000 KRW/day + 300 KRW/min (incoming calls are fee). The hotel charges 10,000 KRW/day + useage.
Sorry, I didn't read OP's question carefully.