Get at least three estimates for each job
Whether it’s the whole job or only one subcontractor’s portion of it, only when you have a basis for comparison will the outrageously expensive estimate stand out. And remember, the cheapest isn’t always the best. If one price is much less than the others, there’s probably a reason why.
Get your job estimated during the off-season
This won’t help in Southern California where construction work isn’t seasonal, but in the snow belt you may well get a better number from a contractor during the idle months when he wants to line up work for the first day of spring. Even if a contractor’s prices don’t vary much from season to season, he’ll almost surely have more time to devote to working out the estimate carefully and maybe even to helping you brainstorm about costs and technical concerns.