Why are the Archives expensive?
"Expensive" is a relative term. Commonly, expensive is used (or mis-used) to imply overpriced. Archives, for reasons that will be outlined, are not over-priced. They are not unusally priced as compared to hardback books that have a lot of pictures in them. They do, however, cost a day's pay if you make minimum wage.
What goes into the cost of an Archive?
The costs of an Archive include the usual costs of book-making (printing, binding, editing, distribution) but with one rather large addition: Art recreation. Each page of a DC Archive edition is a full page of artwork from a comic book, usually between 30 and 60 years old. DC's backstock of art goes back to the 1960's but for material beyond that, the art must be re-created. Re-creating art is the primary expense in creating Golden Age Archives and older Silver Age ones. Hence, regardless of the format (hardback or paperback), Archival collections of DC material older than 30 years are so cost a lot to make. To make the pricetag of the books a little easier to swallow, DC has elected to make a high-quality prestige product that gives the material the status that it deserves. It should be emphasized that this does not significantly affect the price because the art recreation, not the formatting, is what costs. A paperback version would be cost just as much as the hardback.
Are there discount programs available to get them cheap?
A variety of mechanisms exist to purchase the Archives at a price less than the $50 cover price. These are mainly centered around internet shopping venues targeting high throughput booksellers. The best known of these is Amazon.com, the Internet's largest book dealer. Amazon.com routinely offered DC Archives for $34.95 plus S&H. Plus, a little targeted shopping with gift certificates obtained at sites like Amazing-Bargains.com further reduces the price, especially if you buy more than one Archive or combine Archive purchases with other books.
Another very viable alternative is to employ online comics retailers. Silveragecomics.com has a standing offer of $35 each for any Archive or Masterworks edition. I use this source myself and recommend it. Another source is Convergence Comics, which offers similar discounts. Aaron is my primary source for DC Direct but his Archive prices are also in the same range.
Finally, for the ultimate in cheap, use eBay, searching either Archive or Archives. The upside of eBay is that you can save 40-70%, depending on the age and popularity of the Archive. The downside is you're competing for it against the rest of the world. Patience is a virtue on eBay.
Why doesn't DC make Essentials like Marvel does?
The answer lies in the fact that art recreation is DC's primary expense in creating the Archives. Because the cost of recreation is the same regardless of paperback or hardback format, there is no benefit to the consumer pricewise in making Essentials instead of Archives. Furthermore, Essentials generate less income from cover price (since they cost less) and, based on Diamond pre-orders, don't sell much better than the Archives (~65%). Without a big increase in circulation, the lower cover price translates into much less profit. With the big up-front cost of art recreation on Golden Age material, DC would be hardpressed to make any money off Golden Age Essentials.
Making Silver Age reprints is easier, as companies have tended to keep art since the 60's. Since Marvel has only produced 60's and later material, they have little to no recreation costs. They can survive on the minimal profits the Essentials provide. DC's Silver material could be reproduced cheaper as well but DC needs the profit boost to make the whole Archives program work. The larger profit on Silver Age editions balances out the lesser profit on the Golden Age editions. The combination produces a strong and viable collection of a wide range of material that would not otherwise be available. Ergo, Archives it is.