Prices in Asia are including VAT, and are more expensive retail than the US. They get most releases of Nike/Adidas, however there are few places that sell Nike Basketball/Jordan shoes. Most stores only carry runners/casual shoes.
So in general you might find Inikis and NMDs, harder to find ultraboosts. Try shopping in a tax-free shop, which will refund you the vat when you leave the country. Just bring your passport.
Check out Shilin Night market, raohe night market, Ximending station, a lot to eat and shop for in those areas.
I've been to taipei like 8 times and bought shoes there everytime, so I know a little bit about it.
Which store was that? Adidas store? Haven't seen either of those in Seoul. the retail for ultraboosts are so high in Korea though.