The amount paid will vary according to what kind of agreements they do or don't have with insurance companies.

Some insurances will pay more than others. So the docs may get $1800 from some insurances but much more from others.

They set the amount they're going to bill ($28K in your example) at least partly on what they think could be the highest insurance payment they might possibly receive, and then add on a few thousand more.

People who are wealthy and have no health coverage (insurance) will be billed the full amount and will be expected to pay it out of pocket. That would all be discussed with them prior to scheduling surgery with a significant portion of it being paid before surgery.