Is it not reasonable for them to ask, if they don't know?
Also should they not have a general ballpark idea? I'm British and would expect Drs to be aware of costs before advising treatment (ruling out cheaper to rule out things etc).
No, because it will be different for every patient (for the same treatment) and will fluctuate widely from year to year. Also there is a general ethical sense of recommending the best treatment and dealing with cost as an afterthought.
Car insurance prices vary widely from year to year, and person to person. I still expect a somewhat accurate price when I give some minimal information.
Agree with the ethics, but if someone presents with a broken leg do you xray it (cheap) or mri (expensive)? For anything vaguely complicated the Dr is basically just eliminating likely culprits. Its preferable for everyone (that doesn't have a profit motive) if they try the cheap/quick ones first.
If you have a solid isurance provider and an insurer(e.g. google) they work with you to find out the most reasonable treatment and ballpark estimation of the costs. So there is that.