I used the following:
Dollar difference: sum([Sales])-lookup(sum([Sales]), -1)
Percentage difference: (sum([Sales])-lookup(sum([Sales]), -1))/lookup(sum([Sales]), -1)
Sample file as attached. Hope it helps.
In addition to Ey's solution, a number of calculated fields can also be created and placed on the view. I've attached the workbook showing this. Let me know if you have any questions.