You do not apply fixed and variable costs to the WBS. The WBS and its work packages contain project costs, both labor and material. Your business case analysis resides outside of the WBS. I am assuming from your question that the project cost is your revenue, i.e. you are a supplier. I also assume that your contract does not stipulate a separate fee; thus, your contract cost value and your total contract value are equal.
If these are true, then your labor rate that you used to calculate your TCV must include a margin for G&A, OH, and other direct costs. If my last assumption is correct, then the labor rate must also include a margin for fee.
Your TCV is your top line or your revenue. From here, you need to pull in the appropriate amount of allocated costs to calculate your gross and net profit. You need to assume that the margins that burdened your labor rates -- G&A, OH, etc. are the appropriate allocations.