Maintaining an overall profit margin needs to be tied to commission structure and compensation. Information, is the competitive advantage, not mistrust. If you can't trust your sales people with cost information then your sales people must not support your corporate value structure. If they don't support the values of the organization then maybe they don't belong. You see, the competitive advantage, is a human value based structure that executes at lightening speed.
Rapid execution can only happen when there is an open sharing of information.
It is not keeping coast information confidential that is going to give a company a competitive edge. Anyone with a computer can figure out anyone's cost structure on any product within a matter of minutes. But before you can share cost information, or any company information, you must trust your employees and employees must trust each other. And trusting each other goes hand in hand with a human value based organization. It is not easy to change.