A nice round per squar foot charge regardless of material with no setup or minimim or art or base fee is nice and efficent. Like $1 per sq ft or $10 per sq ft. That what your customers want to hear anyway.
But in reality, it has to be a little more complicated. You need to have a base charge to cover customer service, invoicing, management and administrative costs, plus an art charge, plus a per sq ft charge, plus a material charge (like actual cost times a markup), plus an installation charge etc (whatever that particular job calls for).
We do a lot more than just signs, but in my shop, the most dreaded question is "how much do you charge to print shirts". The answer that our customers are hopining to hear is "$x per shirt". But the answer I give them is "it depends on the job specifications, like how many shirts, what type of shirt, what color of shirt, how many print locations, how many colors in each print location, and how long the artwork is going to take to create.
I usually tick them off, but I HAVE to know the answers to ALL those questions before I can give an accurate quote. They have to understand that we custom produce goods, and we dont happen to have their custom made job sitting on our shelf.