You aren't building your formula right.
Worksheets("EmployeeCosts").Range("B" & var1a).Formula = "=SUM(H5:H" & var1a & ")"
This does the same as the following lines do:
Dim myFormula As String
myFormula = "=SUM(H5:H"
myFormula = myFormula & var1a
myformula = myformula & ")"
which is what you are trying to do.
Also, you want to have the =
at the beginning of the formala.