Product History subject area - Custom number on Opportunity

Hi,

We have a requirement to create a report on the Opportunities per Product Category. I created a report on the Product History subject area and included the # of Opportunites, product category and a custom number field we use on the Opportunity to register a specific ammount. There is 1 ammount per opportunity. In the report, when multiple products are related to the opportunity, the ammount is registered per product instead of per opportunity. The report totals therefore give the wrong total ammount.

How can I make sure that only 1 ammount per opportunity is considered?

Regards,

Ingeborg