These are some of the questions that need to be answered, and it will help
you determine the price.
What kind of work? How big? What location? Do you need to research the
theme? How long does it take you to create a personally directed piece? What
are the similarities in the approach to this work? What is their budget for
art? Even though you may calculate the time in linear hours, life happens,
and sometimes the space to create isn't long enough. Break up the project
into small segments that can be done and give a self evaluation to the work,
before proceeding to the next segment.
Materials cost, sometimes you have the materials already, paints, paper,
etc. If you need to buy anything extra to make it. But in general you can
calculate the volume cost of the materials.
Will this new work grow out of the kind of work that you currently do?
Figure out what will keep you emotionally, or intellectually invested in the
project, and push yourself into the uncomfortable zone, to return with a
eureka moment of expression.
The figuring of the final piece is tough, because, if you do it right, the
work will have a life of it's own, and be 'priceless'.
So in the end, go with your instinct and do business.