Doesn't the USA have the most prisoners/criminals in the world? They could force them to build and guard the wall instead of the death penalty/prison time. Lets call them the Nights Watch.
It's a good joke, but to take it seriously: Prison labor actually costs significantly more (like 4-5x!) than just hiring folks in the free market.
Does it cost any more than simply housing them behind bars?
I'm guessing that it requires a ton more expensive staff to secure prisoners that are not constrained by cheap walls, barbed wire, bars, and watch towers.