I must agree with Greg. Nobody needs to know your cost, just your price. While it is risky to shift the parts cost to the labor category (then it becomes a case of "I can find cheaper labor"), hiding your cards on the materials is the key.
Frankly, to net $600 on a house with all of the potential variables scares me. Just one or two mistakes could eat that up in no time. Volume, and plenty of it would be the only way to make a real profit in the grand scheme of things.
