At a vintage shop I picked up a copy of Star Frontiers for $20, only to find that it had the screen, Mission to Alcazzar, and Zebulon's Guide to Frontier Space stuffed into the boxed set as well.
We used to have a LGS in my city that had a good preowned shelf. I managed to buy WEG Star Wars 1e and 2e Revised from them for $7 each. I also bought a good condition Shadowrun 3e softcover from them for the same price. The only book I've found in a 2nd Hand bookstore is a copy of Tales from the Yawning Portal, which appeared to be brand new and selling for $25 - snapped it up and gifted it to a friend.
Picked up an OD&D boxed set (3rd Printing) which included all of the supplements (Greyhawk, Blackmoor, Eldritch Wizardry, etc.) along with a copy of Chainmail and Tunnels & Trolls 1st ed at a charity auction for a mere R400 (around $20).
Box has been reconstructed but the books are all in excellent condition.
Also got a White Dwarf collection #1 to #60 in official binders for GBP50. Got it at the games shop in Aldershot.