Actually Weakness, it is a very reliable way to check your fame because for many the trip to Norg is a very hard one. True, the prices do fluctuate, but if the price for you is around what it should be, you can figure with the inflated NPC price it should be ok. Just use some guestimation.
Oh, I just thought of something, sorry weakness, that update they did where store prices rise exponentially when people buy that item one day. So yea...after SE implemented that it did become harder to determine your Norg fame, but not that many people buy rice balls (or didn't last time I played, maybe they do now), so I think it's still fairly reliable.