Building a sit-stand desk for less than $200

I'm starting to wonder how difficult it would be to build a completely floating desk. Basically, if there was just a tray attached to the monitor arm, and the arm locked in place and was stable enough, you could just raise and lower the arm, and not worry about the desk surface.

I'm thinking one of those floor-to-ceiling bike racks may be strong enough to carry the weight, and sturdy enough not to shake.

Anybody seen a desk like this?

It seems most monitor arms have a weight restriction that is too low for my needs.