I don't know how you would go about using MSOGenerateExcelFile to achieve this : my understanding of that Activity is that it is an OOTB Activity designed for a specific purpose (used within PRPC Wizards etc): so it probably doesn't support the inclusion of Excel Formulas.

As far as generating an EXCEL file using other mechanisms - you could build your own mechanism based on the Apache POI library : this library is included in PRPC.

There is an example here of using Apache POI from PRPC - to generate an EXCEL file.

Another way of doing this is to create an XML file in the (older, but still supported) Microsoft 2003 XML format.

There is an example here of using XML Stream Rules to create Excel documents in this format.

For your particular example: here's a manually created bit of XML which mimics your spreadsheet format:

Here's the XML to do this: (and it includes two other sheets , just to illustrate the structure a bit better: Sheets 'TWO' and 'THREE' are not needed for your example in fact):

<?xml version="1.0" encoding="UTF-8"?>
<?mso-application progid="Excel.Sheet"?>
<ns1:Workbook xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ns1="urn:schemas-microsoft-com:office:spreadsheet">
    <ns1:Worksheet ns1:Name="ONE">
        <ns1:Table>
            <ns1:Row>
                <ns1:Cell>
                    <ns1:Data ns1:Type="String">Price A</ns1:Data>
            </ns1:Cell>
                <ns1:Cell>
                    <ns1:Data ns1:Type="String">Price B</ns1:Data>
                </ns1:Cell>
                <ns1:Cell>
                    <ns1:Data ns1:Type="String">Total</ns1:Data>
                </ns1:Cell>
        </ns1:Row>
        <ns1:Row>
                <ns1:Cell>
                        <ns1:Data ns1:Type="Number">25</ns1:Data>0
                </ns1:Cell>
                <ns1:Cell>
                        <ns1:Data ns1:Type="Number">75</ns1:Data>0
                </ns1:Cell>
                <ns1:Cell ns1:Formula="=SUM('ONE'!R2C1:R2C2)"/>
        </ns1:Row>
        </ns1:Table>
    </ns1:Worksheet>
    <ns1:Worksheet ns1:Name="TWO">
        <ns1:Table>
            <ns1:Row>
                <ns1:Cell>
                    <ns1:Data ns1:Type="String">Cell1</ns1:Data>
                </ns1:Cell>
            </ns1:Row>
        </ns1:Table>
    </ns1:Worksheet>
    <ns1:Worksheet ns1:Name="THREE">
        <ns1:Table>
            <ns1:Row>
                <ns1:Cell>
                    <ns1:Data ns1:Type="String">Cell1</ns1:Data>
                </ns1:Cell>
            </ns1:Row>
        </ns1:Table>
    </ns1:Worksheet>
</ns1:Workbook>

Note: this special XML processing instruction:

<?mso-application progid="Excel.Sheet"?>

This is what cause Microsoft Windows to recognize this XML file as being of type EXCEL when you save the file to your desktop etc.

Note also the format of the formula:

<ns1:Cell ns1:Formula="=SUM('ONE'!R2C1:R2C2)"/>

There the references are not made in the traditional alpha/numeric cross-reference : rather they are in (one-bound, not zero-bound) R and C (Row, and Col) coordinates.

The 'ONE!' reference is for the SHEET.

You can use PRPC XML Stream Rules to create dynamic versions of this XML : you would need to calculate the 'R' and 'C' values as you track through your 'pxResults' page in order to create your formula-references.

You will also need to (probably) set the HTTP Headers to a suitable type - to ensure the EXCEL automatically launches to load this file when you click-to-download.

Even if this all works: you will still need to solve the issue of how to deal with uploaded Excel files of course..... (Which may suggest the Apache POI route [although probably more work initially] may be a better solution - as you can work with the 'native' binary formats throughout).....

Hope this helps.

Cheers

John