We don't. We work on a fixed-rate basis. You have a budget, and nobody likes surprises (well, unpleasant ones at least). That's why, after working with you to define the scope of the project and refine the requirements, we submit a formal proposal including cost and payment schedule. You are under no obligation until a signed agreement is in place.
We also understand that projects can occasionally take unanticipated turns and that requirements can sometimes change. In such situations which deviate substantially from the project specifications, you will be informed of any impacts to the cost and/or timeline and presented with options so that you can decide how to proceed.