I think it's just been the $50.00 I spent on a greenscreen. I guess could also say my editing software, but I was making movies long before I was posting stuff on YouTube...so I'm not sure if it counts. ^^
Maybe a personal question but I am interested. I mean something specifically for Youtube as it says in the title, so please do not put the price of your mac that you probably had anyways then decided to use for editing on.
Other than Dxtory (which I do not use anymore) I haven't really spent anything, other than the extra £300 I just spent on a CPU/mobo just because it would be better for streaming, recording and editing over the one I would have got for just gaming.