Summing large numbers in xslt

Im having a problem summing 2 large numbers in xslt..
Here is a snippet..

<Cell ss:StyleID="s27"><Data ss:Type="String">Grand Total</Data></Cell>
<Cell ss:Index="7" ss:StyleID="s33"><Data ss:Type="Number"><xsl:value-of select="sum(/*/MorningStarFundReport/CUSIP_Market_Value)"/></Data></Cell>
<Cell ss:StyleID="s35" ss:Formula="=SUBTOTAL(9,R[-3]C:R[-2]C)"></Cell>

This works great when the values are
999999.9999 or lower, but any number higher than that does not work.
I am exporting this to an Excel spreadsheet..
Please note that the SUBTOTAL function works just fine

Ive tried to change formatting for the cell, I have also tried to assign the sum to a varibale & work with it..to no avail

Any help out there?

Thanks