With builders, for instance? We're having a new carpet fitted and also getting the outside of our house painted. I'm not sure if the quote they give you is a fixed price or if it is generally accepted as a starting point from which to negotiate?!
I don't want to pay over the odds but equally don't want to offend anyone by asking if there is any wiggle room on price...
YABU - You should pay the price asked.
YANBU - It's fine to try to negotiate the price.