BM Poker World has one of the better deck selections out there. Yes, some of their decks are more expensive than the competition, but some are actually cheaper. I've done price comparisons between them and the1eyedjack - it varies by deck as to whether one's cheaper or they're both charging the same price.
Why are the deck so costly if they're buying in bulk? Perhaps it's because the site owners are running a business and not a charity... It's the cost of the deck, plus the cost of the electricity, the storage space, the computer and Internet services you need to run a business online, and perhaps a bit of profit tossed in. They're often buying at retail when it comes to rare decks, unless they get a Kickstarter to back or cut a deal with the deck company. A quality deck in short supply and high demand will always end up costing a bit more - progressively more the more rare and sought-after a deck is.
You always have the freedom of choice to not purchase from them. If you think these decks can be sold cheaper, then by all means, open up your own shop and see for yourself. I'd love to buy decks at a cheaper rate...