(a) Every state spending unit, as defined in chapter five-a, shall require that every contract or subcontract for the construction, reconstruction, alteration, repair, improvement or maintenance of public works or for the purchase of any item of machinery or equipment to be used at sites of public works contain a provision that, if any aluminum, glass or steel products are to be supplied in the performance of the contract, or subcontract, only domestic aluminum, glass or steel products shall be supplied unless the spending officer, as defined in chapter five-a, determines, in writing, after the receipt of offers or bids, that the cost of domestic aluminum, glass or steel products is unreasonable or inconsistent with the public interest or that domestic aluminum, glass or steel products are not produced in sufficient quantities to meet the contract requirements: Provided, That this article applies to any public works contract awarded in an amount more than $50,000, and with regard to steel only, this article applies to any public works contract awarded in an amount more than $50,000 or requiring more than ten thousand pounds of steel products.

(b) The commissioner of finance and administration shall issue rules which provide that, for purposes of this article, the bid or offered price of any aluminum, glass or steel products of domestic origin, as defined in section one of this article (including any applicable duty), is not unreasonable if it does not exceed the sum of a differential of twenty percent of the bid or offered price of the aluminum, glass or steel products of foreign origin: Provided, That if such products are produced in a "substantial labor surplus area" as defined by the United States department of labor, the differential applied under this article shall be thirty percent.