With the return of some of the TSR PDFs, it is possible to spend a very small amount of money and get a lot of play value with the B/X version of “Classic D&D”. The Basic and Expert rulebooks are $5 each. Ten dollars gives you all the rules you will ever need to play TSR D&D. Sure, you can get Labyrinth Lord (a B/X retroclone) for free (in a no artwork PDF), but nothing really beat the original Moldvay and Cook rulebooks for Classic D&D. A total of 128 pages of rules, spells, monsters, art, and GM advice for both dungeon and wilderness adventures. That’s right, in only 128 pages.
Other than dice, you don’t need to buy anything else to play. However, if you want to get started quickly, you might want to pick up a couple of adventure modules. The traditional starting modules are either B1 – In Search of the Unknown or B2 – The Keep on the Borderlands. Both are excellent adventures and are set up to teach both players and GMs about old school play. B2 is generally the more popular these days. However, I’m going to recommend B1 because fewer people are familiar with it and because it is easier to drop it into B10 – Night’s Dark Terror. B10 was one of the best adventures TSR produced. Designed for levels 2 to 4, B10 provided a sandbox setting covering about 20,000 square miles with a number of adventure sites and dungeons scattered about in this 64 page adventure and room for the GM to add many more interesting locations after the material in the adventure is used up. B1 and B10 are also $5.00 each. So for a total of $20.00 you can get a complete set of old school rules, a starting dungeon and a campaign setting with an adventure. I don’t think you can beat this deal with a stick.