How about just having two level-based measures on the respective grand total level and then subtracting the one from the other. Standard fucntionality.
You create formula giving the total amount on the left, you subtract a formula giving you the total amount on the right and you have your number in a cell. You can then use a narrative view to display it the way you prefer.
Grand total in pivot views are automatically calculated by OBIEE aggregating at the required level the column you have. If you do the same yourself by hand using AGGREGATE(... AT ...) you can achieve the exact same calculation and get by a single formula the virtual total OBIEE shows in a table or pivot view.
EDIT: Or what Christian said if you have access to the RPD
Thanks Christian. Will try your suggestion and get back..
Thanks Gianni, I wili try this option and see if I can the delta in a cell. will test this and get back.