Excel Template - Calculation in XDO_METADATA

Hello,

We are on BI Publisher part of OBIEE 11.1.1.7.150120 on Linux.

I am defining an Excel Template and I have defined two summary fields in XDO_METADATA tab as follows:

XDO_?XDOFIELD18?         <?sum(.//PRICE)?>

XDO_?XDOFIELD19?         <?sum(.//COST)?>

Note that the PRICE and COST columns are on two different tabs in the template.

Now, based on these two summary fields, I need to calculate a % such as:

(PRICE - COST)/PRICE * 100

I am unable to get this to work in the XDO_METADATA tab. What is the correct syntax to be used in this case?

I tried the following, but it doesn't work.