I'd be interested in seeing the sales for each edition. As someone who's played them all, I had to borrow stuff from D&D, being poor and broke. AD&D I got, in all, 3 PHB, 2DMG, 2UA, and about 10 other various books. 2ED, I got 3 PBH, 1 DMG, a BAZILLION books. All the softcover completes, the notebook MM (which includes every supplement I could get my hands on), the 4 Encylopedia's of Magic, and several boxed set adventures. 3.0/3.5 2 PHB, 2 DMG, 2 MM(Both 3.0. I just started adapting my 2ed monsters after that), all the Dragonlance supplements, about 4 of the FR supplements.
The reason for so many PHB in the 1st two editions was that our house hosted the weekly games, and we had a few poor broke players, so we bought extras to lend. I was also married at the time, and we each wanted our own PBH.
Now that my Dragonlance Campaign has died, I don't have a tt group, so the impetus to run out and get 4E is dwindling. I'd still get at least the 3 core if I were going to be playing RPGA, but there are no RPGA's nearby that I can find, so.