No, it's not, because they're not the competition. FFXIV's Online Store items are for your FFXIV characters. You cannot say that you won't buy the items in the Online Store and go to other game's store instead to buy items for your FFXIV characters. That's where the comparison fails.
So the only thing left to see is if enough people would buy those items at those prices. If there is enough people buying, then it's not overpriced. It's priced at whatever the market (of which it has a monopoly) dictates. Even if some of those people would say it's overpriced, if they're still paying for it when it's optional, then their wallet speaks louder than them.