There is a huge difference between the English versions and the Japanese.
In Japan, there was a league incentive of sorts which ran from 2003 all the way to 2006 called the Daisuki Club EXP system. The cards given away in this system totalled 32. While most were given out as presents for things such as re-joining league for a season, others had to be earned via a points, or, EXP system.
You would get points for participating in battles, battle road, and other tournies, and for other things in the league itself, but to amass a grand total of 70,000EXP was quite a feat. Umbreon☆ and the prior level Espeon☆ were the two hardest cards to achieve through this system. Some people managed to amass enough EXP to gain them before the closure of the program at the end of 2006, but not a lot. Thus, what cards did get out and about for sale outside of collections tend to fetch a high price. To boot, they're Eeveelutions. In terms of merchandising and collecting, Eeveelutions are a sort-of hype Pokemon, thus most people will pay exorbitant amounts of money for the more rarer items. The highest I've honestly seen an Umbreon☆ go for is about $1,500ish. I managed to score 'the pair' for $850 through a private sale rather than eBay.
tl;dr - hard as heck to get, limited number, limited distribution, extremely popular Pokemon. They're holo, to boot, while the US PoP Pack prints are non.
Comparatively: Master's Document, the top ranking Action Point prize in Japan at the moment (and, essentially, the Umbreon☆ of the L-P season) doesn't get anywhere near as much. I just picked one up for $320USD today since the Japanese market doesn't over-price them (and the lack of Pokemon on the card potentially helps), while someone on eBay is trying to flog one for 700 Pounds (around $1,105USD).
I hope this helps to give you some insight into why they're drastically different! Sadly, I don't think you'll find most people will trade for them. I know I'm certainly not letting my babies go after such a struggle to get them at a decent price. Haha
