I just bid a remodel job where the specs. called for NM with metal boxes. I wasn't happy with my bid price, though so I ran the job again using assemblies consisting of NM and plastic boxes. I was really surprised at the difference in labor units and material cost between plastic and metal. the metal boxes made my project cost considerably more than using plastic boxes.
Another option that I like to us is the Allied Moulded boxes, I think a lot of guys hate them, but I have always liked them. They are more expensive than blue boxes, but can make your company stand out above the competition if you sell it as a better quality installation, which is probably what the builder is looking for anyway.