Still beats it
No, it doesn't. I have to build it using an hour or 2 of my time to do so. It sounds petty but I make $50 hour OT rate so I'd be losing money. If you're just looking to be technically right, have at it. But let's at least try to be a bit realistic.
With that said, I would by no means ever consider building a computer to save myself $40. Hell I wouldn't do it to save myself $150. The margin for a build would have to be about 20% or at least $200 or more for me to consider it worth it.
In 2011 I built the computer I'm currently on and added minor upgrades overtime. This build saved me $400. The idea of saving an hours pay to build something instead of just buying it off the shelf doesn't really scream "cheaper" to me.