Looks like for your money you get a typewriter (presumably refurbished), a display and the electronics. As you noted, he sells a DIY kit if you want to provide your own screen and typewriter (with an estimated assembly/install time of 6-7 hours for an experienced person.
Actually sounds like a fair price to me. But perhaps you are used to getting your electronics assembled overseas on a massive scale by (essentially) slave labour.
Display? No. Those are iPads in the Etsy store. The typewriters are essentially USB keyboards. For $600-$800 you get an old typewriter and an pre-wired-up Arduino board. You're paying for the integration of this http://www.etsy.com/listing/62642931/diy-usb-typewriter-conv... into the typewriter.
Actually sounds like a fair price to me. But perhaps you are used to getting your electronics assembled overseas on a massive scale by (essentially) slave labour.