What contractors really make per hour?

[Tyco Bronye]

I have heard a few times that it's better to pay contractors by the project and not by the hour because it takes away incentive. That being said, based on this pay per project system. I'm wondering how much a contractor is making on average per hour? and how does it vary with each trade?

Plumber?

Electrician?

Window guy?

Dry Waller?

Hardwood floor installer?