Our current auditor, one of the 'Final Four' - gave us the choice. They would leave us a set of workpapers, or retain them for us. Our prior auditor, another of the 'Final Four' didn't give us the choice - they keep them but we have access to them.
While the bottom line is pretty hard to swallow, the cost of doing audits in-house and keeping up skill levels of employees, etc. is a pretty large number too!
Opinions my own.