Valuing Sub-Subcontract Costs

An instruction was issued to a Facade Sun-Contractor to expend a defined provisional sum for the design, supply and install of secondary steel to support curtain walls. The Subcontractor sub-subcontracted the labour element of said works to an external company and simply issued their quotation + there own OH&P as the cost for these works. Having known the install gang size and the duration / location, the rate seems extremely excessive. Is the Contractor obliged to pay the quotation + OH&P. I would argue that had we of known the Sub-Contractor was going to to sublet the works we’d have tried to agree the costs upfront, However this has never disclosed or requested by the Sub-Contractor. What is the best approach to agreeing these costs.