How do I create a calculated field that is the division of two sum data
fields? (i.e., two cells in my pivot table)

The field list when going to calculated fields does not show my data fields...
For example for the below I would want to calculate (sum of price)/(sum of
Balance) for each CategoryXFuture Category cell. How do I do that?

Future Category
Category Data A B C
A Sum of Balance 23.4 25.7 49.0
Sum of Price 10.3 21.9 10
SumofPrice/Sumof Bal. ? ?
?

B