How detailed is the painting contractors estimate? Is is based on time and materials? Or is it one price? Does the estimate break down the work by the various rooms to be painted and the necessary preparation? Or does it not provide any detail?
In order to evaluate an estimate, you need enough detail to really understand the work that you are paying for. On the one hand, you can pay a painter to paint your room where they'll simply throw a coat of cheap paint on the wall. And on the other hand, you can pay a painter to wash your walls, fill in and smooth any surface imperfections, use primer, fill and smooth any remaining imperfections, screw down anything loose, caulk cracks and trim, tape and drape plastic and drop cloths to ensure your floors and other surfaces are protected, and paint using the finest quality paint and painting tools. On the surface, these two painting jobs may not look all that different. But as you look a little closer, and over time, you will see a huge difference. In short, you get what you pay for.
So ask for details and understand what you are paying for, and you will get a sense for the difference. Also check references to get a sense of the painting contractor's work and reputation.