ws8n, there's no problem getting a taxi back to the Grand Hilton late at night. The cheaper cabs will cost about 10,000 won from the Namsan area (that's a little more than $8). The subway costs 700 won (58 cents), but it stops running before midnight. There's a station about a 7 or 8-minute walk from the hotel.

A regular (cheap) taxi from Incheon to that hotel will run about 50-60,000 won ($41-$50), depending on traffic. That can be reduced about 50% by taking one of the frequent and comfortable buses that run from Incheon airport to Kimpo airport, then a taxi (there should be plenty of taxis waiting in a queue there).