As a general rule, if you're given a price for something by a company that specializes in that thing, and you think you can do better, you're wrong.

The creator alludes to this when he says not to ask how long it took him to build the machine.

As a corollary to this rule, if you've estimated your costs to do a job, and a company says they can beat it, you should probably do the job yourself. I bought a 3/4 ton jointer four hours from my shop, got a quote to rent a lift gate truck and a quote from a moving company/rigger. They were about the same. There's no way they can pay someone for a minimum of eight hours of time, plus cover their truck and fuel costs for what it costs me to rent a truck. The delta on their truck costs can't possibly be enough to pay someone good for eight hours.

Don't your rule and corollary disagree?

This is one of those things that's incredibly context dependent. There are lots of fat cat small companies out there who do easy tasks with thick margins. There are also jobs that are deceptively hard which it makes sense to hire out.

I usually try to err on the side of diy, but everyone has a different threshold on these things. Sometimes the economics work out when you don't count your time.