You have to have overhead, if you're operating legally.... even if you ARE a Police officer.
You still have licenses, proper insurances in place, electric, heat, paper towels, chemicals, tools, proper ventilating for your printing system and other hidden costs. If you don't have these in place, you should probably arrest yourself.
Regardless if you're operating out of your bedroom, garage or a commercial building... you still need to be set up by the IRS rules, city ordinances and proper insurance policies to be considered in legal operation. Failure to follow these few steps is..... as I'm sure you must know..... or you wouldn't have said your overhead is so low, grounds for a codes inspection to insure your neighbors, family, dog, environment are all safe from your type of business in your locale. One cannot ignore the law.... right ?? I know I can't.
Part-time or full-time.... you still need to be kosher 100% of the time.
Yes, get a square foot price, figure your basic design time along with production and application time. Add this to your overhead, markup and profit columns.
The other thing you can do is like so many others..... just name a price, do the job, collect the money and throw it all up in the air and whatever falls to the ground, you keep..... screw anyone else that has their hand out for your hard earned money.