Cards for the have been printed in both English and Japanese since 1998, but not all cards are created equal. The card flipping market has always existed to some extent.
But it's taken off in recent years, catapulting some of the rarest cards to astronomical prices. As a result, many collectors have looked into buying Japanese cards, which, much like their English counterparts, can become precious collectibles in their own right.
A number of factors determine a card's value: its rarity, its condition, and its usefulness in competitive play. Japanese cards are subject to all these values — but because they're produced and sold in a different kind of market, Japanese cards aren't always worth the same exact amounts as their English equivalents.
It varies from card to card, but here are some of the factors that influence the value of Japanese cards, as well as some examples of direct price comparisons. Most Japanese cards are worth more than their English counterparts, simply due to their rarity in English markets.
Still, the value of any individual Japanese card is affected by the same factors as an English card. It's impossible to say with any kind of general certainty whether Japanese cards as a whole are worth more than English ones.