It's helpful if you let people know as early as possible if you're going to go over the estimate by a substantial amount. In the example in the grandparent comm

It's helpful if you let people know as early as possible if you're going to go over the estimate by a substantial amount. In the example in the grandparent comment, I can understand why the client would be unhappy paying $30,000 for a project that was estimated at $15,000 if they didn't find out until the very end. Imagine if you had to get your car fixed and when you went to pay the bill it was twice as much as the estimate, and compare that to the experience of getting a call part way through to inform you that the fixes will be more expensive than originally estimated.