Is it possible to customize the Task cost calculation?

Thanks John-project, this helped a lot. I read the articles you suggested, and understand better the differences between Task, Resource and Assignment data now.

I agree with your suggestion, the Cost Rate Tables seem indeed the natural solution.

I'm now trying to wite a macro creating the Cost Rate Table B for the EC hourly rates using the raw hourly costs of Table A:

Sub AssignHorizonEuropeCostRatesToCostRateTableB()
    Dim projApp As Object
    Dim resource As Object
    Dim costTableA As Object
    Dim costTableB As Object
    Dim rateIndex As Integer
    
    ' Create an instance of MS Project
    Set projApp = CreateObject("MSProject.Application")
    
    If projApp Is Nothing Then
        MsgBox "MS Project is not installed."
        Exit Sub
    End If
    
    projApp.Visible = True
    
    ' Set the active project
    Set ActiveProject = projApp.ActiveProject
    
    ' Loop through all active resources
    For Each resource In ActiveProject.Resources
        ' Specify the resource cost rate tables
        Set costTableA = resource.CostRateTables("A")
        Set costTableB = resource.CostRateTables("B")
            
        ' Table B rates = (Table A rates) * 1.25
        For rateIndex = 1 To costTableA.PayRates.Count
            costTableB.PayRates(rateIndex).StandardRate = costTableA.PayRates(rateIndex).StandardRate * 1.25
            costTableB.PayRates(rateIndex).OvertimeRate = costTableA.PayRates(rateIndex).OvertimeRate * 1.25
        Next rateIndex
    Next resource
End Sub

It's nearly working at the exception of a type mismatch error in the last couple of instructions (I'm completely new to VBA, this was largely inspired by Copilot), which I should be able to fix.

But the next step then will still be the creation of a custom field Cost1, which will use the Table B rates instead of the Table A ones. Would that be possible? I haven't found how.