If the total cost including incidental expenses of any acquisition or improvement, less any amount to be paid toward the cost from any source other than special assessment upon the parcels benefited thereby, when added to the aggregate totals of all assessments and estimated assessments as stated in the report, exceeds in total amount one-half of the total true value stated in the report of all the parcels to be assessed to pay any part of the cost of the acquisition or improvement, no part of the excess shall be assessed upon the parcels.