The cheapest way is, of course, by salvaging materials. Concrete blocks or bricks can be used to make the sides of the desk, and a trashed door or shelf can be used for the desktop. You want the desktop surface to be very stable and sturdy, so make sure it's not hollow, and be sure it's thick enough to support whatever's on it. (I would make it strong enough to sit on.) If you want to make it out of purchased materials, then get 6 letter/legal size storage boxes and one piece of 5/8" plywood that's 24"x48". Stack the boxes 3 high and place the plywood over it. You'll need a barstool for this desk, but it works, and the boxes can (and should) be used for storage: the weight of stuff in the boxes will help secure the desk.