Note: This Rollup Helper use case comes direct from a customer! We love it when our customers submit their most significant use cases! If you have a Rollup Helper Use Cases, please submit it to marketing <at> passagetech <dot> com. Submitting a use cases allows everyone to get the most from their investment in Salesforce!

The scenario is that users want to review their pricing for each product to
make sure their margins are in the correct range. Because Salesforce
doesn't expose the Price Book Entry to the Reports tab, you can't create
such a report. 

Customer Use Case
Create a Rollup that rolls up the List Price from the Price Book Entry to
the Product. In addition to using a custom Cost field on the Product, 
you'll need to create a custom Price field on the Product that acts as the
target field for the rollup, along with a Margin % formula field to
calculate (Price - Cost) / Price. Our case is simple because we only use
Standard Price; if you have multiple Price Book Entries per Product then you
will need to get creative with filter criteria on the rollup, or make an
arbitrary rule such as rolling up the MIN value.