There are three kinds of “best purchase”, particularly when it comes to 2nd-hand RPGs.
There are the books that I got for a steal, relative to their value to me. For me, the tops of that list would probably be most of the Providence supplements, some of them acquired at 75% off.
Then there are the books that I’m glad I got at the price I did, because I would’ve overpaid if I’d paid any more. Usually these are real stinkers that I have just for curiosity’s sake—the Actor’s Book of Characters for World Action and Adventure is laughably bad. I’m genuinely curious to some day get a look at the core rulebook, but I can’t imagine paying more than a buck or two for the privilege. To quote the summary linked above, “The amount of detail included is astonishing, but unfortunately little of it is relevant to play.” I think that’s a fair assessment—much effort, poor results.
But also in that category are games that I feel I should know about, even if they’re not my style. So I wait to find them on the cheap. I bought a boxed set of the D&D4E core books so that I could refer to them, even though I already knew I would never play it—but not until I could them at ~50% off. Similarly, I’m eternally proud of getting a slightly-battered copy of GURPS 3rd ed for $6, back when it was still the current edition and nobody was talking about a 4th ed. I had already suspected that GURPS wasn’t for me, and reading that confirmed it. As a game for me, personally, to play, $6 was paying too much. But after D&D and WoD, it was probably the most-referenced game at the time, so as a game designer it was a steal.
And then there are the game books that I’ve actively stalked and finally found. Rare books that are hard to find, regardless of the price. For me, the two at the top of the list are Alma Mater and the original Bunnies & Burrows. (Well, it’s the 3rd printing, so there are some significant revisions, but it’s not the GURPS version, which I have yet to actually see.) I don’t remember the circumstances of the B&B purchase, other than that it was a very reasonable price. Acquiring Alma Mater is an interesting, if short, tale:
I was working my way through a booth at Gen Con with a lot of old books, some a little old, some a lot old; some common, some rarer. I wasn’t the only one, of course. Someone else pulled out a copy of Alma Mater, a game I’d only ever heard of. I must’ve oooed pretty loudly, because he turned to me and said “you clearly need this more than I do” and handed it to me. And on top of that, it was in almost-new condition and under cover price! Yay for geek solidarity!