If the rolls are made before the contracts for the construction of the improvement are let the total amount of such rolls shall be the estimates heretofore referred to, to which estimates the county road commissioners or the state highway commissioner shall add such further sum, not exceeding 10 per cent, as shall be deemed necessary to cover contingent expenses. If the rolls are not made until after the letting of the contracts for the improvement, the county road commissioners or the state highway commissioner shall take the contract price for such improvement, to which they shall add the incidental expenses to the time of making the roll, and to this total such further sum, not exceeding 10 per cent, as shall be deemed necessary to cover contingent expenses. Such rolls shall show the total sums exclusive of interest, which shall be paid by the county, the townships at large, and the lands benefited, and also the sums that may be payable hereunder by any cities assessed at large.