Before going ahead with adding resource costs (or others) make sure the schedule is as correct as you can make it.
Get feedback from all interested parties.
If necessary give everyone a breather and review it a little later with fresh eyes.
The ‘second review’, shown below, only has personnel resource levels and no costs.
Consultation with the finance department will afford average personnel costs for the major resource blocks identified.
These figures can then be entered into the calculations to give the final budget cost plan.
Whilst this may have some links with cash flow there will not be a direct correlation as cash flow refers to the time when money is actually paid out.
Consultation with the finance department should allow you to more accurately present the costs in terms of cash flow.
This costing may well establish the final budget.
Once agreed it may be difficult to try to justify additional funds as no more may be available.
Make the costing of the stage plan, and its schedule, as accurate as possible.
One of the constraints may well be budgetary.