I certainly wouldn't recommend lying on your costing sheet, these people see this kind of thing all of the time and would be able to work in a minute whether you have over inflated your figures. Usually you would have to put the pay rates at the right amount, what I mean about adjusting the costings is not to lie about your costings but make sure that when you do your calculations on materials, equipment, PPE, administration, travel etc etc you put them through at the most they could possibly cost, this will probably be true for the first three months of the contract then costings will come down so you will make the saving there and of course your 'margin' will the % on the figure on paper making it slightly raised in true terms.
Hope that hasn't confused you!