IMO, square foot pricing is a great way to go out of business quick. It isn't bad using something like the Means Construction Cost Data to check an estimate, but I would never risk my business bidding by the square foot.
Most shops around here develop a flat rate price for each type of device & use that number. This is the system I use & it works well for me.
You might try checking the business forum since this sounds like a likely topic that may be covered there.
Few things are harder to put up with than the annoyance of a good example.