Can't find the specific price for MTG japanese cards
While it is sometimes the case, not all foreign cards are actually worth more than there English counterparts. Nonetheless, older cards definitely might be. 

While you can check eBay, this is likely your best source:

http://www.hareruyamtg.com/en/

If you can't find a reference anywhere, check other foreign languages of that card, such as korean or russian.