Luxury has some scientific definitions. Top of the list is size for a single-family dwelling. My city has liberalized zoning for size, but kept limits on number of occupants and such for fear of impacting infrastructure. The result is ever-larger units. New houses seem less like houses and more like diagrams of local building codes. Property line minus ten feet, there's your wall.

Luxury apartments can also be defined during zoning as units that the city assumes will rarely be occupied. It is often easier to get a permit if you can claim that the people in your development aren't going to contribute to traffic problems. I've even seen the pre-selling to overseas investors trotted out as justification for not upgrading traffic connections to a development. So-called "elder living" units also fit this scheme.

As for rents, the standard has always been that yearly rent should be around 1/20th of the unit's value, the "20-year" rule. But with property prices rising so quickly, some units are renting out at 1/50th their value. Once you get into those areas, rent becomes irrelevant and you run into the paradox that many units (houses and condos) are worth significantly more without tenants. You don;t want to bother asking tenants to clear out so you can show the unit. So investors planning to flip something in the next couple years don't bother with rent. Only when they think that they may have to hold on for a while do they consider.