Depends on the size. Large uploads are limited to wifi to lessen the load on the network same as downloading large Apps.... If you are on wifi when you upload it doesn't use any data. If your on 3G/edge you are charged as Per your data plan.
You should also look into options which allow you to upload HD video directly to YouTube. iPhone 4, by default, down-converts before it uploads to YouTube, but there is an app called PixelPipe that allows 720P uploads to YouTube, Vimeo, etc.