Its so expensive because the target market for a digital back is not the photographer, its the rental houses, studios, techs, and wealthy landscapers. The rental houses, studios and techs are making about $500 a day on their backs, almost everyday. They make large sums of money from their backs, which are often leased, purchased at the end of the lease for a $1000 or less, then sold as "Off Rental" for a modest discount off new, or traded in. They buy in bulk and get them cheaper as well. Same with the new Profoto Pro8.
The wealthy landscape guys, well, they want the best and will pay for it. Over and over. Never making a penny but they don't care, they've drilled teeth all their lives so whatever. A $42k back is about half of that Porsche they just bought.
Who would you rather sell a back to? A photographer that bitches about everything, or some guy who just wants the best, at any cost, and then upgrades to the next back as soon as they are out?
The makers don't make them cheaper because they don't really have to, although that may change. They have been coming down in price, but still . . .