Relevant costing

Hi,
The question says that
The labor is fully employed by….on jobs, where they produce contribution of 2$ per lab/h.
Their current rate is 10$ and extra could be hired on 20$ if necessary.
I understand that this 2 dollars are lost opportunities and thus cost to the business. But why 10$ is a relevant cost. In any case we are going to pay them the sum.Could you please help me to understand where my mistake is.

Thanks a lot.