This part of the bidding process is where a lot of game playng takes place.
It seems that there are many contractors who deliberately bid under their costs, relying on change orders to create a substantial profit later on. I suspect these guys don't inform the customer of the necessary changes before the job is awarded.
Another tack is to visit the job you lost, and look for ways to demonstrate that the guy who did get the job is in some way not meeting specs- then pointing that out to the customer, hoping to financially punish the other guy.
