Revenue based from Project Assignment (hr)

I am trying to get the expected revenue based on the project assignment (hr) that is manually entered in the resource panel (see image below). How can I do this? Or is there a formula I can use to reflect this on my report? I tried using this formula:

$LoadOfUserMonthly.RevenueRegularRate * $LoadOfUserMonthly.LinkObject.ProjectAssignmentBillable

However, there is an error: Data types Currency and Duration not compatible for '*' operation