This may be a limited thing, but I see the Swords & Wizardry Complete Rulebook up on RPGnow/DTRPG for “Pay What You Want”. This is normally $34.99.
Written by Matt Finch and Published by Frog God Games, I’ve really been meaning to read through this. I may be a bit wrong, but from my research, regarding retro-clones, S&W is to be based on the 1974 OD&D. And, this seems to have a lot of support.
S&W comes in three versions, an essential “White Box” version, an updated “Core” version with expanded rules, and this “Complete” rulebook which I gather combines all the above.