The reason some of the Mogstation items are overpriced is because China or Korea made them, they set the price high and Square Enix has to comply with that price.
Just hop onto a Korean MMO and the first thing you will find is that you're basically playing $30+ per month on items and services.

The community keeps asking for Chinese/Korean items, well that's the price they set for their items over there in those countries.
if it's up to Square Enix on the other hand they give you fairly priced items for things made in Japan like $6, $10 or around $12-$15. Anything $20+ is China/Korean made.

PS. Yes they're made in Korea/China, the quality of what they make is lower than those made by Square Enix themselves. Just look at the princess dress and the cloud mount.