Get enough bids so they cluster.
The true cost of your job in your area today is probably bid c or bid d.
Company a is using substandard materials or labor and
company e is looking for suckers.
If you get bids identical down to the dollar it may be price-fixing. Prices off the Web will be probably be low-balled.
Low-Ball - Wikipedia, the free encyclopedia
Some advise telling each bidder that you are getting other bids. I don't usually tell them. If you do tell them and you see the smile disappear, I'd eliminate that candidate.
Or take a look at RS Means [publisher] books on construction costs at your local Border's or Barnes & Noble's.
Cost of Replacement Windows - Get Prices and Estimates - CostHelper.com
says your median household income is higher than the US avg. of $50K then you can expect to pay more.