Looking at that price, these guys must be out of their minds. For the $340 they claim you would spend printing the parts, you could probably buy a 3D printer, the filament needed, and pay for the power to print it all, and still have money left. Maybe that's what they meant. $200-250 3D printer, $50 of filament and probably less than that for power. And all the parts can't cost much more than $150. ~$50 for a capacitive touchscreen kit, $20 or less for the battery, $35 for the Pi, maybe $10-15 for a lipo charging/5V power board and the rest on the keyboard. I recognize that keyboard. None of those parts are expensive, it's all cheap Chinese crap.
All in all you might pay like $500 to DIY it if you don't have a 3D printer already, but half that cost is the 3D printer, which you can then use for any future projects you may have or to print little useful objects to save yourself trips to the store or ordering online.
Of course, there are people who don't want to do it themselves and just want something prebuilt, or easily assembled, but even for those people, there are way cheaper options using similar parts.