Subcontracting cost caclutaion per square meter area not Per Unit

HI experts,

I have a subcontracting process in one of the sales order for which cost calculation in routing maintained is $20 per unit.

As this is a variant configured material, so one of the characteristics is surface area. the Product is a Door and it is sent out for coating to subcontractor and they now want to charge on per square meter rather than Per unit Door charge.

How can I fulfil the above requirement