It's like the NHS, a lot of the cost would go to agency workers. If you brought all these people in house, you'd probably get better results, but then you have to pay them year round (or tinker with Fixed term contracts), which is why agency staff can be returned when no longer needed.
I woul.d speculate you'd need half and half. Half to stay permanently, and half for the summer peak. as to the exact figure that would be...thats anyones guess