Until February 22nd, 2015, there’s a sale of Wizards’ D&D pdfs at DriveThruRPG.

The discounts are variable – The World of Greyhawk Fantasy Setting (AD&D 1E) is $8.49, down from $9.99, while the D&D Rules Cyclopedia (D&D Basic) is $4.99, down from $9.99. Every title is at least 15% cheaper.

Note that the “cover price” is the price the book was originally published with; a lot of the books tend to sell cheaper than that as pdfs.