Looks like @mimighster already mentioned and linked all the threads I would have linked. Not too surprising, since I’m the guy who helped him recently in this thread. 
As already mentioned, some Japanese UED cards are much rarer than their 1st edition counterpart. Especially for the E series, L2/L3 sets, and the World Championship Pack.
Almost all Japanese sets (ignoring a few minor exceptions) will be harder to find in UED than 1st edition. For their prices there usually isn’t a huge difference, unless the sellers know what they have in their hands. Today I received a lot of about 20 cards with some UED E2 and E3 cards for which I only paid 5 USD including shipping, but I’ve also seen an UED E1 card sell for about 75 USD because the seller (and buyer of course), knew it was a special promo release.
Still looking for two UED World Championship Pack cards to complete my Seviper collection, and also still need the UED E1 Pikachu. Luckily I do have the UED E3 (Skyridge) and World Championship Pack (EX Power Keepers) Pikachus already in my possession. 
Greetz,
Quuador