My guess is (I don't know the actual statistics) that sets such as MoM, and all TT blocks are cheaper and easier to find because the game was at its highest popularity, and thus in its highest print, which means there are more sealed cards in general still around for lower prices. I stopped paying attention to LOTR TCG after block 10, so I can't say for certain how popular Expanded was, but I would assume that it started to decline in popularity and thus print, which means it's a lot harder to find. The vendors aren't necessarily selling their product at those inflated prices (maybe they are and I'm just oblivious), but that's what it's "worth".
I typically use eBay myself, or try and find some local "barn finds" at 5 belows, or Kmarts and the like. If I'm really trying to hunt something down, I'll shop around on Amazon, eBay or other card retailers that pop up in a google search. Prices are typically compatible, but the more you hunt the better deals you can find.