$800USD!? Unfortunately, if you're paying this much for a tablet, you're probably going to be paying correspondingly higher for electronic components, as well.
What generally makes it more expensive for you to roll your own electronics is that you're not buying thousands of components and cutting deals with manufacturers or their distributors. This is true for things like PCs, even though they're mostly built from off-the-shelf components.
That aside, the biggest cost, since neither form factors nor sizes are standardized (as far as I know--this is a selling point for various manufacturers), you're looking at doing a lot of custom work. Not just for the enclosures, but even the motherboards themselves, which you'd have to design, fabricate, and populate. Unfortunately, a large number of the components will probably be ball grid array or otherwise be extremely difficult to hand-solder (assuming you have the experience and education to do the routing necessary to make the PCB in the first place).
Not a bad idea, but the game has changed quite significantly since the Steves (Jobs and Wozniak, of Apple fame) were able to build cutting-edge computers out of their garage.