I just did a dummy booking on Agoda.com to test your theory.
Hotel booked in KRW was priced at 125,413
In USD was 106.00
In EUR was 95.00
So Agoda is estimating a KRW/EUR rate of approx 1320. The mid-market KRW/EUR rate is 1328 or so.
Looks to me like the Agoda rate is right where it should be. I fail to see the "scam".
(You do realize that Agoda adds any taxes and/or service charges after you select the hotel, right? So if there's a 10% service charge, it's added at checkout - which may account for why there may appear to be a differential from the price you initially saw quoted and what is actually charged.)