I'm buying FAR less than 3.x* for 3 reasons:
1) I'm playing less, which in addition to making me sad, makes it harder to justify the spending levels I used to maintain in 3.x.
2) I know WotC probably doesn't want to hear this, but DDI is good enough that it makes many books unnecessary for me. Yeah, they are starting to put more sidebar and additional info in some of the Power books that isn't in the Compendium/Char Builder, but those haven't been enough for me to justify buying any of the Power books. Plus I only bought AV1 because the GSL references specific page numbers. AV2 hasn't much caught my eye. Oh, and as a slight aside as for the campaign settings, FR 4e held no interest for me, and for Eberron 4e I bought the Player's Guide as soon as I could, but haven't bothered with the Campaign Guide, but again, there's not enough I don't already have to justify the purchase - although in that case it's 3.5 overlap instead of DDI.
3) Far less third party support. Still some great stuff out there (although sadly less now that One Bad Egg closed up). But in the height of the 3.x era, I had Malhavoc, Green Ronin, Bastion Press, Paradigm Concepts, Fantasy Flight, and later Paizo all taking bites out of my paychecks - and that's just print books and doesn't even touch the massive number of PDF publishers I purchase(d) from. Now, not a single non-WotC 4e physical book has caught my interest, unfortunately. Some great PDFs, but that's about it.
* My spending in 2e and earlier really is hard to compare since most of my pre-3.0 gaming experience was high school and earlier. I mostly took a break in college, and then as a bona fide grown up got back into it not too long before 3.0.