The moogle mount for $30 is a 2 person mount. They figured they could charge more for it because of this. Lot of people have purchased it so it is not overpriced for many people.
As for the Chinese dress it is priced the same as it was in the Chinese Mog Station, they just converted the price into US dollars.
As long as people with pay for the items they are not overpriced, when people cease to purchase the items then they will lower the prices.