This will give confidence to the leadership in decision making. If we talked about money everyone would tell us we can't do it. We decided not to have a budget because we are afraid it would depress or scare people. What really scares people is NOT having a budget. If not told otherwise, the congregation probably imagines the project will be beyond their means.
Some people avoid setting a budget because they fear it gives permission to spend the full amount.
In reality, if a firm ceiling is not known, we will likely design plans for a building which will cost twice our limit. We would never consider shopping for a car without having a price range in mind. When purchasing a home we surely determined first what we could afford. It is unwise to approach a church building project any differently. First we will determine what we can afford.