In my painting business, I always felt crappy after having a sales call and spilling my guts, telling a customer everything I knew about the job, spending hours talking about product knowledge, techniques and such. And then giving the customer the estimate, only to call back several days later to ask the outcome of their decision and to find that the client would not give me the time a day. Needless to say, I felt betrayed, and really angry.
After reading several sales books and going to several sales courses, I heard others speak of the same type of behavior.
They called it being an unpaid consultant. The idea is that all clients or customers and potential customers want information and are willing to do anything to get it except commit. They will go out of their way to tell you oh, you stand a pretty good chance. We like you a lot are probably only get the job. None of that is true for the most part; those lies are just a technique to get you to pour out your heart. It's kind of the same feeling you get when you give the client the estimate and they're standing or sitting there with their hands grabbing and grasping for the estimate. Even when they do not know what is on the estimate. I always felt like I was robbed.
The best way to counter this is to be in control of the conversation. If you're on the measure call then the only talking you should be doing is asking questions. If they ask you a question, calmly answer that you'll tell them everything when you come back to give them the estimate. It's also a good way to ensure that you get invited back, because after you spill your guts, and once you give the price. You're done; they do not want to hear anymore.
Controlling the conversation, you do not allow yourself to be pulled in. When they ask you a question turn it around and ask it back, or some variation of the question. Handling it this way, you gain more power. This way when you're sitting down with the decision-making people, along with your estimate, you control the situation and can calmly show them the benefits of going with you.