Answered Jan 20
Depending on the exact situation, it can been taken as extremely rude.
It comes very close to simply asking them how much money they earn. While some people live far above or below their means, most people have a strong connection between income and rent (or mortgage) payment amount.
There can also be a tone of competition, where they are afraid that you might be able to afford a nicer place than them. At the other end, they may be offended if you pay less than them (i.e. you are under less financial stress).
When the person asking has been a new coworker, or some other random person whom I just met, I consider it very rude. When people immediately ask this type of question, it indicates a very intrusive attitude in general. I have had a few instances of people thinking that they had personal authority to dictate the location, floor-plan, and price of my apartment.
A related, but less obnoxious issue is when a new person immediately asks about neighbourhood or suburb. Sometimes it’s just making conversation or looking for commonality. Other times, it is a very conscious socioeconomic question, including judging you as either a degenerate, or as a snob, depending on where you live.
The one exception that comes to mind would be among low-income university students. In my city, there are some student housing providers that have standard prices published online. Also, some students have a sort of camaraderie around being poor, and having modest housing.
Generally, I avoid intrusive questions of acquaintances and coworkers, in any area of their personal life that doesn’t directly affect me.