You have lots of great points there KB, but English cards prices are quite stable. Surprisingly I'd even say. But you are right, even for them it's not possible to offer fixed prices that would be accurate. It's just the nature of every TCG, the market value changes based on supply and demand. The supply for DM-10 - DM-12 cards and promos is very limited and the demand relatively high, so estimating fixed values for those cards is especially difficult. Nonetheless, since Duel Masters TCG is discontinued outside Japan, the market is not really facing anything new.
Global market places like eBay, Amazon and Carousell help keeping the prices about at the same level no matter where your actual location is. This doesn't really apply for OCG cards since the language (even alphabetical) and cultural barrier between most of Asian countries and the rest of the world.
What comes to the higher value of non-English cards, I have to disagree a bit. Not many duelist still accept the idea of using non-English cards on their decks and the collectors really prefer English cards if there are no cards available in their native language. And even then, they usually prefer English cards since they are more universally accepted. This way the demand for English card is far superior to the demand of the cards printed in different language and thus although the supply for some language editions is very limited, the prices really are lower. This is true at least for the German prints, since the supply for them is one of the highest of any language edition there is for DM. Back in the day English BurgerKings promos went for around 60e while the German ones went for around 30e each. But the value of German cards at least seems to be rising again, based on what's happening on German eBay. Spanish cards, though, aren't really no where to be seen. French and Italian prints are sometimes offered.
Since Duel Masters OCG is still being printed and new cards come out, I wouldn't dare to make a fixed list like that for them. It would be very inaccurate even after just one week or so. But on the other hand Japanese values are easily observable by inspecting the Japanese Marketplaces, mainly Yahoo Shopping and Yahoo Auctions since they usually are offering almost any card that would be relevant in their meta. Other cards, like very old first editions, not so much. So the original OCG prints could be even more valuable than their English prints, since there surely is more Japanese collectors nowadays than others since the game is continued only there.
My point being, the values of Duel Masters TCG cards are quite stable, unlike the values of OCG cards. Usually English prints are valued the highest but this is all determined by supply and demand.
It's true that there isn't really misprints in DM and they usually are very minor. I haven't even seen any of those so called Artist Proofs at least not a DM one. Haven't even seen a photo. They surely are rare!