A couple of points. A lot of the discussion here has focused on the resolution being 4X that of 1080p. IMO,
@Mr_Sukebe hit the nail on the head.
4K uses a far more efficient CODEC than 1080p, and so in terms of data storage and streaming bandwidth it's about 2.5X the bandwidth required to store and send UHD. However, because it's a different CODEC (H.265 vs H.264) then I'm guessing that the services do need to store files in both versions since there'll still be some devices around that can't decode H.265.
Honestly though, I believe that these costs are a minor consideration. The driver here is supply and demand. Services charge what they can. For example, Sky still charges extra for HD at a time when nearly all new TVs are 4K.