As I mentioned yesterday, the core rule books of Dungeons & Dragons are the Player's Handbook, Dungeon Master's Guide, and Monster Manual. You could always get all three of them together for under $100, and that was enough to get a full group playing for years. Nice for poor students, but not really a recipe for a huge financial success of the game company. TSR was nearly broke in 1997 when they got bought by Wizards of the Coast. And while I don't have details, I always thought about that purchase as being a case of "I liked the product so much, I bought the company". I am pretty certain that WotC made a lot more money with Magic the Gathering and Pokemon trading cards than with Dungeons & Dragons.
But of course the fact that you *can* play Dungeons & Dragons very cheaply doesn't mean everybody does it. In some ways D&D resembles a Free2Play game, where especially the Dungeon Master can buy all sorts of nice convenience items. In my enthusiasm of getting back to D&D (and having foolishly thrown away most of my old stuff years ago), I bought quite a lot of rule books and rule supplements, adventures, and especially everything having maps and monster tokens. The stuff hasn't become any cheaper since I was a poor student, but my financial means have improved over the last quarter of a century.
Much of what is on sale is targeted at the Dungeon Master. There are campaign settings, adventures, and monsters galore. The players aren't supposed to buy those, except possibly for the "Player's Guide to" the campaign settings. Buying adventures at best spoils a player's fun, and at worst leads him completely astray because he thinks he knows what will happen while the DM often enough changes the story and only uses the parts of an adventure he likes.
But what I noticed especially with the 4th edition is that there are now more and more books printed for players. There is now a second and a third player's handbook, plus several "player's option" books. All of these books contain new character classes with new powers. I've counted 26 character classes just in the Wikipedia entry on 4E classes, and I think the character builder on the Dungeons & Dragons Insider website has even more. There are also lots of races added to the game this way.
I must say I am not a big fan of buying new rules to increase the number of character classes. So many classes mostly end up being confusing and it becomes difficult to roleplay the differences between them. The deluge of player's books seems more designed to increase WotC's revenues than to make Dungeons & Dragons a better game. And as it is difficult to see how strong a new character class is on paper, I would be cautious to allow a player something very exotic that then turns out to have been min-maxed for power rather than chosen for being interesting. I rather support Wizards of the Coast by paying for a subscription to Dungeons & Dragons Insider, which has a lot of useful tools for Dungeon Masters.