Textures for V4 are still pretty much 100% original work as not many merchant resource packs have come out.   Once texturers start using those as a base for their creation and thus not having to do all the seamwork themselves I think prices will go down.   Additionally,  V4's mapping does make it harder to make seamless textures, limbs are separated from the body, ears are huge compared to the head and on the body map making it hard to match their texture size and seams to the head's. 

Another point to consider is that some V3 products have gone down in price since vendors would like to get as much out of them before they become obsolete so that could also account for the difference in prices.   Of course others in the past have just pulled their products for V1/2 rather than drop prices.

I'm still waiting for my favourite merchants to bring out V4 products and for the UTC plug-in that will allow me to convert V3's textures to V4, so I'm holding off buying anything for now :)