Easily £500+ ($1,000+) on WOTC and 3rd party materials.
I'm still undecided on 4e - the volume of materials for 3.5 and other systems (CoC, IH, C&C, D20M, Conan, SW, Traveller, and WFRP) that I own are certainly a factor in this, but it's mainly that I'm not sure I like what they seem to be doing with the apparently 'broken' system.
That said I'll definitely be getting the initial core books so I can investigate the new rules properly, but I have yet to be convinced to switch.