Summary fields can be used to produce sub totals instead of grand totals if you place them inside sub summary layout parts. I can see that you have already done this to get an employee total.
If you also add a sub summary layout part "when sorted by" the rate field and place a summary field in that added layout part, you will then get a different subtotal for each hourly rate value for a given employee. The key is to then sort your records by employee, then by rate.
If you need one row of data per employee for each rate at which they worked, you can replace the body layout part with this sub summary layout part that I have just described.