Roughly $715, so I voted "501-750"
(I'm rounding a few dollars up or down....yes, I actually looked at the MSRP on each of my books, but I don't have that for the DM Screen and Character Sheet pack that I got, which I guesstimated and included in that total)
And that's only counting purchases of WotC products for 3E, excluding my 5 books for d20 Modern, and excluding the 6 third-party books I got (of which only 3 are support for 3E, the other 3 are core books for similar d20 games). And I excluded whatever I spent on issues of Dragon Magazine and Dungeon/Polyhedron Magazine.
Altogether, the magazines, d20 Modern books, and 3rd-party stuff probably totals another $700 roughly, the vast majority from the magazines (I never subscribed, I just picked up issues that interested me, but there were a lot....and newsstand prices are a lot more than subscription prices).
.......and yet, it's not so much what I've already spent on d20/3E that makes me loathe to bother with 4E. It's 4E itself that makes me loathe it.
I'd continue buying books if I thought they were worth using. I stopped buying D&D books altogether when 3.5e came out, with only 2 exceptions, the Tome of Battle and Player's Handbook II. I didn't find 3.5 worthwhile, just as I find 4E to be even less worthwhile, because it won't even be reasonably backwards compatible with 3.0, unlike the ToB and PHB2.