one thing that you (and many others) seem to be missing a little bit is that the most expensive external component is the screen, and in many cases that does not need to be a dedicated device, instead it can be a HDTV, i.e. anything with an HDMI input
I also disagree with you on the amount of storage needed. You don't need hundreds of gigs of storage, tens of gigs can do very nicely, and that is available today on phones (and will be even easier/cheaper going forward)
Ubuntu can be pretty comfortable in single-digit Gigs of space for a fairly complete installation, it's media files that eat up so much space for most people
At that point, external storage becomes a performance option, not a requirement, and the only 'dedicated' peripheral that is needed is a keyboard/mouse set, and those are pretty cheap, even if you want a wireless set, and dirt cheap if you get a Android device that will let you use UDB versions.
I fully expect that there will be more expensive dock sets out there, but the question will be what is it that you are looking for? if what you want is a laptop/phone (like the Atrix), you will pay a premium for it.
but if you want the ability to work the same way both at home and at the office, it should be pretty cheap.
I expect that the first round of manufacturers will be pushing this capability with proprietary docking stations and the associated prices, but I don't think it will be long for some of the other manufacturers to make cut-rate versions that just use standard peripherals