I think the $20-$25 is what a plumber gets paid as an employee.
If you call a plumber to your house, you are engaging a plumbing business (even if a sole trader) so you pay $75 an hour. The mark up covers cost of ute, tools and the risk that you can't get 40 hours a week work and a profit margin. An employee at $20-$25 does not carry any of these risks (but obviously has risk of being retrenched).
