For example if I wanted to know the price for rice in Japan, where could I look it up?
check this website
I'm pretty there isn't one site to look up any product, but there are specific price indexes you can view.
For example, the global big mac index : http://www.oanda.com/currency/big-mac-index
Actually, that's only one I can find I'm afraid. There's a global "food" price index too, but it's not for specific items.
Though I can tell you the price of Rice in Japan is extrordinarily high to due national laws that prevent the import of cheap rice and make sure rice sold within Japan is from Japan only. It's just one of the many corruptions in modern day Japan.
If you need to compare the price of electrinic items, you ca use PriceGrabber.com,Price runner, etc. You can finad a lot more by searching google using the keyword "Online Price Comparison+Itemname" or you can use Google Product Search. You can check some cool search engines described on http://coolefriend.com.