Custom Time Phased data field based on Staffing Requests PLUS Project Assignments

To achieve some PS forecasting, we were wanting to make a custom field that would take the Time Phase Data of Staffing Requests + Project Assignments for a Project and multiply the Sum of the Hours with a calculated dollar value. We are not seeing a way to sum both Staffing Requests and Project Assignments in a Custom Field. Any advice on how we can achieve this?