In addition to the iterative approach, you can also offer a base rate and accept all the tasks, even if they're not quite perfect, then pay a bonus to the worke

In addition to the iterative approach, you can also offer a base rate and accept all the tasks, even if they're not quite perfect, then pay a bonus to the workers who really nailed it. That way you're not rejecting the workers who are making an honest effort, but the better performers are being paid accordingly.