The only effective arbitrage I can see here is via USD/KRW; however this must be heavily regulated in South Korea creating effective foreign exchange controls. Otherwise this arbitrage would have been done a long time ago. It is USD/KRW that is out of whack here.  

Edit 1: The non KRW price should be considered the true free market price here.
Edit 2: If effective foreign exchange controls is not the issue, then there is something seriously not right at Bithunb with KRW fiat withdrawals.