I don't understand how people can be liable to pay a bill they have no knowledge of upfront, like not even a rough estimate. Even a plumber will tell you "well

I don't understand how people can be liable to pay a bill they have no knowledge of upfront, like not even a rough estimate. Even a plumber will tell you "well, I bill $150/hr and I think it will take 2 hours, but who knows". Is there any other example of an industry that operates like that?