Addition to quote

Contractor miscalculated his bid – used 44 squares but we now need 64. Said he would increase bid by his material cost. To me that means 20 add’l squares of CertainTeed Landmark Hi-Def (I live in 77079-Houston). I don’t need to know his cost, but it can’t be more than $120 per square! That would be $2,400 – not $5,000. What am I missing? Is he not adding it at cost or is the double cost attributable to labor? Would that make sense to you (if that is his excuse)? What rule of thumb for labor to install the additional 2,000 sq ft? Not trying to cheat my contractor, but must understand the math. Thanks.

John