The average dealer in the U.S. makes 2.75% on sales. That means if he sells a job for $5,000, he makes $137.50.
We would recommend you receive at least 2 bids on a job, if possible, but in the heat of summer, or in a cold snap in winter, this may be very difficult, as the dealers have all the business they can handle and don't want to waste time just bidding jobs.
The key to making sure you are getting good value for your money is to use a manufacturer recommended dealer through their dealer network. Most of the complaints we receive are from homeowners that received more than 3 bids and then took the bid that was significantly lower than all the other bids. The only way a dealer can get a lot lower than other dealers is to cut corners. We find that these dealers are usually not in business very long.