Both of the currently existing responses answer the question well. This response looks specifically at some of the costs associated with modern book publishing.
Book publishing is a massive industry, and it takes a significant number of people to bring a book to market. There are authors, editors, marketers, executives, administrators, accountants, lawyers (copyright & otherwise), HR departments, clerical staff, printers, supply chain logistics companies (distribution centers, warehouses, trucks, etc.), individual booksellers, and more. Each of them is integral to the process of getting content from the author's thoughts to the customers' shopping carts in a modern economy, and each of them needs to cover their own costs and make a profit, so that the people involved can pay their mortgages and put food on the table. Think about the number of people necessarily involved in producing a title that sells several million copies and is eventually translated into multiple languages for distribution around the world.*
When we do find cases of contemporary resources being provided gratis, it usually has been made possible by a sizable donation contributed by a wealthy individual or a financially successful ministry, sometimes in the form of the author's own ministry selling the item at or below cost. As economist Milton Freidman was fond of saying, "There's no such thing as a free lunch" -- that is, even when something is nominally "free," it took work to produce it, and somewhere, somebody either donated their own work, or payment was made by a benefactor on behalf of the end recipient.
It's also worth taking note of the extent to which the "e-reader revolution" -- the Kindle, Nook, etc., as well as fully functional tablet computers -- is causing massive disruption in the publishing industry. Profit margins are being decimated, there's an ongoing antitrust investigation into Apple and Amazon re price setting on e-books, and on and on. Several magazines have recently moved to online-only format because it's the only way they can stay viable in the face of competition from blogs that have next to no overhead and can fund their efforts entirely though advertising; SmartMoney Magazine is one recent example. Online publishing has effected Christian teaching ministries in that it has made it possible to provide resources to the public "for free," while incurring significantly fewer costs than would be involved in publishing the same material in print form.
All of that being said, I agree with Mike's statement: Many of the best resources are long out of copyright, and are available "free." See, for example, the great resources available at the Christian Classics Ethereal Library, paid for by the generous folks at Calvin College. The majority of the resources there are centuries old. It says something about the quality of a book when it is still being read and studied hundreds, even thousands of years after it was written, and in fact, we still know many of the authors by name -- Luther, Calvin, Spurgeon, Augustine, Aquinas, Josephus, Tertullian, and so on.
* If it's not too far off topic, here's a fascinating video where economist Milton Freidman explains how free markets encourage peaceful cooperation between many different people to create products that none could produce on their own: The Lesson of the Pencil. If you watch the video, just think about a book instead of a pencil; the principle still applies.
