So far, I've been buying about the same number of books as 3e.
I bought most of the initial 3e books (say, 75% of the first 20-30 WotC releases), then dropped to about 50% for the middle of 3e, then cherry-picked about 25% of the last third of the releases (e.g. the Fiendish Compendiums, Magic Item Compendium, etc).
The same is true so far for 4e. I can see myself avoiding Martial Power 2 and similar. I suspect I'll buy PH3, given what I've heard of it so far. DMG3 will be a harder sell - it will need to include some really good crunch, some advice for Epic campaigns, plus a compelling setting (e.g. City of Brass). Anything less and I'll survive without it.
AV3 is similarly on the margin: AV1 was awesome, but primarily for the new weapon types, alchemical items, etc. AV2 was hurt by being "only" magic items, and I wouldn't buy another simialar book. I'd like to see more weapon types, more masterwork armors, more involved rules on item creation, more rituals, more alchemy, more mounts and vehicles, and support for building strongholds and siege equipment.
MM3 is a must-buy, as is anything on the Planes, new "specific" monster books (Open Grave, Draconomicon, etc), and any items that are steeped in nostalgia. I don't buy 4e modules, but I'd probably purchase a "Return to... [classic module]" adventure. The latest Giants module doesn't count: it may be a homage, but it's just not the same.