It was a few yrs ago for me but standalone boutique was cheaper for me since they gave me some form to turn in at the airport to get refund on the tax. And if I remember correctly, the base price is just higher in Korea duty free stores, vs US pricing even after the US tax priced in. I never really shop at duty frees in Korea for that reason, for non Korean brands. If you are based in US, I'd recommend avoiding duty free shops for luxury goods in Korea. Plus usd to krw conversion is very good right now, so even more reason to avoid dollar pricing in Korea duty free.