Hmm i didnt shop that much, probably abt 100,000 krw only. (I changed at about 920~951 krw to 1 sgd in korea). I brought 1000 sgd there, and changed at the airport there, used roughly about 900 sgd (air tix to jeju, accom, food, transport, pubbing, shopping, sightseeing, concerts). I totally ignored the international brands there, since shopping for those in usa would be much cheaper. I went for the small korean boutiques/shops found in the dongdaemun area, which is somewhat similar to queensway shopping center. A lot of the branded stuff (LV, hugo boss etc) there are fake, so i ignored them anyway. The good thing is that they are open from like 11am to 5am! So you can go around in the day sightseeing, and go shopping there at night! Though you need to take note that some of the shopping malls are close 1 day in the week, so need to plan in advance!
All the shopping malls in that area sell mostly the same stuff (doota, migliore, hello apm etc). Personally i prefer doota shopping mall the most, a T-shirt there probably cost 10,000 krw, a long sleeve shirt cost like 12,000 ~ 30,000 krw, and a blazer cost like 45,000 ~ 55,000 krw, all depending on how well you bargain. A lot of shops there sells long sleeve shirts and blazers, and some of the material used is quite good, and the cutting is quite trendy, quite worth it to buy though its "brandless". As always, try to walk around and ask about the average baseline prices for such items, best is if you can offer a price so low that the shop keeper totally give a black face and bo chup you, and slowly work your offers up at different shops, then you can hazard a guess about their actual baseline

and usually cash will give you a better discount since they dont need to pay the credit card fees...