OK - Imagine that in 8 hrs you can usually earn £150 - £200 for yourself.
If your round is full you might decide to only sub out rarely for £200 plus costs.
If you earn £100 - £150 but have many empty days you might sub out for a week at £500. Especially if you are paid quickly.
It's up to you really.
My round is full and I'm not looking for work so the rate has to be at the top end of what I earn to make it worth the bother.
And being paid at the end of the day is more tempting than two months later when the customer pays the invoice.