1. "I think I'm worth $75/hr"
2. "the job will take 40 hours"
3. "I'll charge $3000 for it"
but rather, like this:
1. "I think they would be willing to pay me 40k for this, and I think the project should take 40 hours"
3. "Therefore, I'll negotiate for a $1000/hr pay-hourly contract with a projected end-date of two weeks."
Simple: Charge $21k per month. If the company drags it for a year then you make $252k.
The granularity here is entirely arbitrary. Hourly billing is by far the industry standard. That's why we default to it. It's easy to justify in court, it's easy to reason about contracts which use it.
If you're billing hourly, and the client chooses not to use your time, you don't work any hours and you don't get paid. If you then billed them for 100 hours without working, you've committed fraud.
If you're billing monthly, you've set aside the month for the client. If they choose not to use your time, you still get paid, because in your agreement you specified "use my time it or lose it".
If he had tried to quote $21,000, he would have gotten thrown out the door.
What if the project was billed at a fixed cost, he negotiated $21k, but the project took a year to complete because the company moved so slowly? That'd be a terrible salary. He'd have to quit and somehow bill even with no deliverables. How hairy would a contract covering that be to defend when you sue?