It depends on the quality of the paint. I paint mostly office space in south Florida and have learned through trial and error that if you use cheap paint, you get cheap results, and spend more money on labor applying more coats of cheap paint trying to achieve quality results. The bottom line is that you have to spend money on quality paint if you want a good finished product. The best paint I have used is Benjamin Moore Satin Impervo. It is an oil-based paint and costs around $50 per gallon. Latex paint dries too quickly and shows lap marks. Oil-based paint dries slower and levels out; it is much more forgiving. And it is a beautiful finished product.
Ultimately, you get what you pay for. Spend $20 on a gallon of paint and you will end up ramming your head through the door, which will cost you even more money to replace it. Or you could spend money on the real thing, put two coats on, and wonder why such a thing as latex paint even exists (for doors and trim).
Figure roughly $8 for paint per door. Labor is always negotiable. I know guys that charge $70 per door. I charge $50 per door for commercial office space, including the cost of paint. For residential work, it depends on whether or not you have any beer in the fridge.