Actually I can understand why a "per foot" price would be a good sanity check in residential even if you do have a more complicated bid process.
I would suggest an "adder" for the extra bathrooms (and kitchens if they have them). Other than that you are really just complying with 210.52 in most homes and that is a constant plus a sq/ft adder.
The open question is "low voltage". That could be anything from a little stub of bell wire from the demark location, poking out of a wall somewhere to an extensive structured wiring system.
