There are sign people that do have tens of thousands of dollars tied up in their machines, programs, and frankly years of experience. If you under price what you do, then word will get out, and you may be busy, but you will be taking work from the people that depend on their line of work to put a roof over their head and feed their families.
So now on to your original question. Wholesale prices for unfinished work (in Cedar or HDU, depending on thickness) is $45 to $60, finished $60 to $75 psf.
I charge $125 per square foot for a finished dimensional sign, using signfoam 3.
What ever you charge, make sure its a fair market value.