Look at the detail included in the proposal. Has the builder really looked in depth at the project? Are all aspects of the work included? Can you clearly see how the builder has come to that price? Generally speaking there is a real median price for any job, which you could expect variation of about 10% up or down from. Outside of that there is usually an issue with someone not understanding what is involved, or overpricing because they are busy, or under pricing because they are quiet. If in doubt, especially on larger projects you can pay a small fee to have an independent estimating service provide costings for you. Which I am more than happy to arrange if requested.