I think a lot of clients offer estimates based on how much money they have to spend regardless of the complexity.
It's hard to give a quote when there are no detailed specs yet. Most clients still want a price tag. Of course afterwards they'll nail you on this premature conjecture.
As a business your quote needs to be competitive and still be profitable.
From experience, we usually throw things out and create boundaries in our offers / quotes. We've learnt the hard way that often times clients implicitly assume certain features to be there without ever communicating them to us, so you need to think ahead and have it in black and white that if they want X they have to get another offer / quote at later stage.
So imo it's not just about what work a quote includes, but also what work it does not include.