Using Formulas in a Report
Summarizing on a numeric field allows you to do simple sums, averages, etc. But what if you want a more complex calculation?
For any summary report - you have the option of adding more complex formulas
In Summary Formats - you'll see a folder in the field list called "Formulas" double click on Add Formula to create a new formula field
This opens a layout to create a custom summary formula
Create the Formula
Add Operator and Number
See your Results
At first you'll just see the summary. But hit RUN report and you'll see the dollar value of service summarized for each opportunity and a grand total at the bottom!
Edit existing Formulas
There is a lot you can do with Summary Formulas and Salesforce has all the help you could possibly need. To learn more about operators, formulas, and functions, Log into your Salesforce instance and visit Salesforce Help: Building Custom Report Formulas