Well, there of course has to be some addendums for add-on's etc. "If the customer keeps moving the goal posts, you can never score the touchdown" is what my old boss always said. And yes, bidding is tough, that's why you see so many jobs with higher than normal costs. Unfortunatly, that's the way it is. We (the company I work for) always say up front that it could be up to 20 percent more in case of unknowen circumstances. Most of the time, if you can prove these cost overruns, the customer will not argue your charges.

As far as system design, salesmen should NOT be designing systems unless they came from the field. They essentially no idea how all this stuff works in the real world. Every company should have an engineer to do this for them, along with CAD drawings of the system.