From the contractor's standpoint: did he submit a quote for you to approve? Because he can come back at you and say "hey, I offered a quote, and you are responsible for looking at it and questioning whether my estimate seemed high".

I used to woed for a company that ordered office cubicals and we would submit quotes that listed how many connectors, corner pieces, etc to build the cubicals. If for some reason, we submitted a quote that was short or over on parts, it always fell on the customer because they ultimately approved the layout and quote. Now.... I've NEVER seen a customer actually lay out the plans and count how many widgets and whoozits they needed, but the company said they offered a client the opportunity and it was the client's problem that the product was ordered incorrectly. Not that I agree with that mentality, but its the way they looked at it.