As above what would you pay to rent work 
Is there a standard price per hundred pounds worth
No standard rate but I used to supply tools/van/water and get 60%. Not as bad as it sounds because most of the work was priced higher than my own anyway.
It began to stop working for me when the company I rented from went VAT registered but only raised about half the prices. My 60% of the take became 60% of the price without the VAT. No problem on the ones where the VAT was added to the customers price but a decrease in income for me when it wasn't.
I only mention this as I have now rented work with two different companies and each time issues arose that hadn't been considered when striking the deal. No-one's fault. It's just the way it was.
When striking such a deal it's a good idea to think it through and discuss things that you masy not have envisaged without that chat.
Things like: Who loses out on bad debts? Is the rental price the price without the VAT? How long before payments are made? Is any money to be withheld?
The first time I did this, I found that I only got paid for the work where the customer had paid. It made it a nightmare matching my invoices to the cheques. If they wanted to withhold any money, on reflection I would have preferred a fixed amount, pay the bills as invoiced, then make any adjustment when the deal was eventually terminated. Much simpler for the accounting. Of course it also meant that if a customer didn't pay, I didn't getpaid.
The second time I did it (with another company), I was very insistent that my invoices were paid whether or not the customer had paid.
In both instances, I would invoice for the 60% but show the settlement figure as 60%-cash collected. That way it kept the invoicing straight and also meant that I had some, albeit reduced, cashflow.