if you think about the cost of getting caught with any amount of pot, and the scarcity of herb, and then factor in the community that buys herb is almost exclusively made up of people that won't really feel a $100/gram hit and finally that same community usually is from a place (like LA or NY) that they are accustomed to smoking regularly, then people will pay 100/gram - and thank you for providing it for that price at the same time.
People that buy in Korea are either a) visiting or on vacation and even though they'll pay a fraction of that cost at home...they're NOT at home and sometimes that lil somethin somethin will make a good vacation GREAT or b) they've just moved there after growing up in a place like the US that has a 'pot culture' and a taste of home is sometimes just what you need to make an adjustment as big as moving half way across the globe into a society as foreign as the language. And unless you're growing yourself, having a consistent supply is near impossible, so its not like you're going to your guy every day or week or month and grabbing a few grams and chillin and in a couple days see your guy again. its more like grab what you can because you just don't know when you're gonna smoke again. I mean it seems unreasonable to you because you have access at your fingertips and really what's the worst that'll happen to you for getting caught smoking? a slap on the wrist? over there, getting caught in a smoky room even without any herb physically there can land you in prison for a year! Put yourself in a country just about as far as you can be away from where you've grown up, in a foreign culture, speaking a foreign language, and you come across a guy that can get you herb at 100/gram...you gonna say no?