Many contractors simply know how long it will take to paint the room and multiply the hours by a numerical factor. They also will account for the time to estimate, discuss the project, driving time, set up and clean up time, paint store time, etc.. to get a real idea of how many hours this project will take. It might take you 4 hours to paint a large room, prep, etc.. one man. But, add in all the other hours to go bid the small job, paint store run, etc.. you might find it is an 8 hour project instead of 4 hours.
You will want to calc your hours by hourly rate. The hourly rate should compose several factors including wages, payroll insurance and taxes (employer and employee side), insurance, company profit and overhead. Add it all up and make sure you are comfortable with the bid and submit it. Don't forget to add in the cost of material and supplies.
This is a very basic approach that works well on repaints. Than, you can get more sophisticated and do some division to arrive at a square foot price. Next time you have a very similar paint project you can measure square footage and multiply by your factor.