It's not that they don't know what they want. It's not that they change their mind. It's that they are oblivious to the cost (to you) of both. Furthermore, when the cost is fixed there's typically no incentive for them to be reasonable.

I find it funny how often I hear, "it's just..." Really? You hired someone else because of your lack of knowledge/experience but suddenly miraculously you have complete clarity on what it's going to take?