Get three to five estimates on a major project, two to three for the smaller jobs. Fewer estimates won't properly compare products and prices, and any more than that is unfair to the contractors that take the time to bid. Selecting a contractor based on lowest price can lead to a catastrophe. If one bid is seriously lower than any other, ask that contractor why. It could have been a mistake.
Most contractors purchase their building products at relatively the same price.
Ask the bidding contractor to double-check his specifications and his price. Ask the contractors who had higher bids why they would charge more. Sometimes their reasons can make enough sense to sway your thinking. Be prepared to make changes in your life and accept the fact that every improvement project will have minor problems and setbacks. While almost no project goes exactly as planned, if you take the time to choose an experienced contractor with a reputable track record, you can be fairly certain everything will work out fine.