So I was reading this earlier, which try to explain a rationale for how rapid gentrification of Manhattan (really a kind of ULTRA-gentrification, since in many areas it is now the professional classes being driven out by the financial-international elite) has actually been beneficial to the city.
I don’t see much of value in the argument really. But it did get me thinking about the ways in which cities are hostage to things outside their controls. Sure zoning policy, and the failure to adequately expand housing stock, have contributed to New York’s rental problems (Don’t worry though! Surely the rush of massive luxury condos in Midtown will alleviate the problem!), but there’s also a lot that’s beyond the capabilities of even the most pro-growth New York administration (and Bloomberg is far from that..). New York cannot alone resist the tide of rapidly rising income inequality in the West since 1980, of which New York’s banker billionaires are symptom, not disease.
More importantly, I’ve often wondered if demand for Manhattan was artificially inflated because there’s only one. At least in the United States, there is no substantial urban space with a density level and city tone of Manhattan. Only Manhattan has Manhattan’s amenities, cultural institutions, etc…
But what if it wasn’t alone? One way to increase residential options in Manhattan to build more units there. But, you could also achieve that goal by building MORE MANHATTANS. We could have five or six major hyper-dense high-amenity large urban spaces.
Which, with any luck, might bring down rents.
The Viking Dutchman 
How has China’s multi-, mega city expansion effected the costs, amenities, etc. of those new cities?
It’s an excellent question! Unfortunately China doesn’t provide anywhere near the kind of data that would be necessary to really dig into the issue. Not to mention the difficult question of how the internal passport system impacts things…