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...