Everybody has a different business model. For the average commercial/electric sign shop that installs and services electric signs (paying union or prevailing wages) the number is probably closer to $250,000 per production employee (journeyman or equivalent). Office personel get paid less, and that comes out of overhead. Owners don't get paid unless they are out in the field doing the work. For a small commercial shop that pays lower wages ($10 - $25/ hr. limited benefits), with lower general liabilty and workmens comp premiums and much lower operating expenses, a figure of around $125,000 is probably more accurate. Specialty shops and artisans will have numbers all over the place.
Signs of the Times used to publish a "State of the Industry" report that surveyed this information. Not very useful, because every business is different, but at least it was a stick to measure by.
One guy I know who works by himself out of a small office/shop bills about $350,000 annually, and keeps around $100k before taxes. Most production and installation is out-sourced, but he does capture a significant part of his income by charging for design and project management.