I think I wrote that before but just one word of caution.
I stayed at the Grand IC (dec 2002) so I don't know if this applies to COEX.
The booking was made through IC's website and the rate quoted was in USD. When I checked out, the rate was in KRW which was reconverted in USD at a terrible rate.

The result was that the amount charged in USD to my credit card was higher than the actual rate booked. After a few email exchanges, the rate was deducted but then the USD was stronger against the yen and I lost money all the same. I hope others don't get the same problem.