In my opinion, pricing a house by the square foot is a good way to lose your shirt! When I was in business we charged by the outlet. We would then add extra for the service, appliances, heating and cooling systems, etc. We would also add extra if the house had cathedral ceilings, was built on a slab, or other factors that made it more costly to wire. Keep Time and Material on a few jobs and you should be able to come up with an accurate "per outlet" price. Remember there are no shortcuts! Taking the time to do a thorough estimate leads to satisfied customers and increased profits. Quick and dirty "per square foot" pricing leads to the opposite.
We did one home where the owner wanted a receptacle about every six feet rather than the required 12 feet. He would have been pretty upset if we had doubled our square foot price. Since he was paying per outlet, he knew exactly how much he was adding whenever he requested an outlet above the code minimum.
If you want to include light fixtures, give the customer a quoted fixture allowance. Then if the customer wants super expensive light fixtures, they will understand that they will need to pay the difference.