Bear in mind when large construction companies are bidding large construction projects their methods of estimating are very far removed from those of us small, 1 or 3 men shops that are doing small fabrication work.  Bidding done on a large project is guided by Project Management factors that really do require a Bachelors degree in PM at a minimum to understand.  It goes way beyond time and materials and markup and employs complicated strategies and business algorithms that are generally beyond the needs of small shop owners and as such beyond their understanding.  One small example would be to look at the RS Means Cost Data which you might locate in your local university engineering library or try a free weeks trial online to access.  Means national cost averaging on a steel stairway is something like $516 per riser, thats w/out handrail or paint, several years ago I  was very surprised to see a 6hour time estimate for a 6" sched 40 pipe weld in one of their books.  Surprised because as a welder I was thinking if it took me that long to make that weld it would be the last one I was ever doing for that contractor.  The other posters point that weather/location/geography play a role is correct.  A 30% efficiency factor needs to be considered when facing inclement weather as per RS Means.  Familiarize yourself with developing a schedule using the Critical Path Method with a little google searching.  The research will likely give you a headache and visualize it's application to something like a HZIG Boiler Construction project with a 500 man workforce.  It gets complicated real quick, is a regular cost of a large company's overhead and keep in mind they will have multiple projects going simulataneously.  See if you can find a price on a 30" heavy wall 9% chrome 90 degree fitting and get it posted.  My WAG is it's gonna be around 10 grand for starters, but for fun, let's find out.  A don't forget to specify foriegn or 100% domestic.