I know in some Japanese manga/anime, characters have held up fingers while vaguely saying "it will cost this much", and the fingers represent multiples of ten thousand (Japanese and Korean are ten thousand based unlike English which is one thousand based). I'm thinking it refers to 700 x 10000 won or 7,000,000 won. Another alternative would be multiples of the largest bill (which is 50,000 won right now), but I think that's less likely since that is more transient and less intuitive than the numeral system. Or maybe they already translated it and he really is excited about 700 dollars a month. Not sure, though. (You can roughly estimate won to usd by diving by 1000, and yen to usd by dividing by 100, though it varies a bit over time).