The problem is just the economics of the matter for any sort of business/entrepreneur. Buying 'cheap' items from overseas just isn't going to be cheap, unless you've got a friend doing it at cost. The price of the item is irrelevant, the issue is that you have to add enough on the top to compensate for handling/processing, plus margin on top for profit seeing as how you had to acquire it to begin with.
Presuming its not a hassle to acquire, somewhere in the range of 15-20 dollars on top of whatever the cost of an item (this would be shipping, handling, profit, etc, lumped together) is pretty reasonable. It just gets less and less reasonable looking the cheaper the item initially is. The amount of costs/profit involved don't really go down just because the initial cost of the item is lower (well, until we start talking large items and freight and blah blah blah). I have seen used copies of FF13 go for about ~40 shipped on ebay, so that seems to be about in line.
Naturally this changes somewhat if the seller is a used game store, has constant stock on hand, doesn't need to purchase the used game at its sale price because they already bought it off of the former owner directly, et cetera. But, again, the market for used japanese games outside of japan is probably very small, so I don't think that exists.
So you're stuck with buying from people who sell at a markup from a typical used store price/native auction price OR finding a buddy in japan who will proxy bid for you and ship it at cost because they're a super cool person.
I don't live in japan. I'm not that person.

I'd offer if I was!
Edited: 2010-04-09, 1:00 pm