We had 2 card keys, but on one of the mornings couldn't find the 2nd one so asked for a replacement. They told us we would have to pay a penalty charge on check-out if we couldn't find it by then. I didn't ask how much this would cost. They seemed serious about this charge though. However, by the time our final day there came, we found the missing key so paid nothing extra.

I was not surprised to be honest as I've come across this elsewhere both in Japan & Korea...seems that lots of "budget" chain hotels in asia have this penalty clause set up if you lose a room card key. I know for a fact the ibis budget hotel (also in Busan) charges 10,000KRW per lost room key (that's about 9 USD or 8 euro).