Adding numbers from an array into a single cell

Be careful, you have trailing space in B98 and the code will look for that row.

Sub test()
    Dim x, myAreas As Areas, myArea As Range, txt As String, myCols As Long
    With Sheets("Payback Calculator")
        x = Filter(.[transpose(if((b1:b1000="Start Date")+(b1:b1000="Costs of Development and Operation")+(b1:b1000="Revenue Contribution "),row(1:1000)))], False, 0)
        If UBound(x) < 2 Then
            MsgBox "Missing something": Exit Sub
        End If
        Set myAreas = Intersect(.Rows(x(0) & ":" & x(1)), .Columns("f")).SpecialCells(-4123, 1).Areas
        For Each myArea In myAreas
            If (myArea.Count = 1) * (Not IsDate(myArea(1).Value)) Then
                txt = txt & "," & myArea.Address(0, 0)
                myCols = Application.Max(myCols, myArea.CurrentRegion.Columns.Count)
            End If
        Next
        .Cells(x(2), "f").Resize(, myCols) = "=sum(" & Mid(txt, 2) & ")"
    End With
End Sub