While the same "but how I would afford the same lifestyle in US" might be a concern for middle class immigrants from poorer countries considering getting there, it's not really relevant to mass immigration from poverty stricken areas, as people from there can (and do) enter the US and live in dirt poor areas, working whatever minimum wage job they find (in fact they're welcomed from employers for being able to get paid so much less), and taking it up from there if/when they can...

it's also true that many people from poverty stricken areas in other countries live in moderate to expensive areas in LA County, like LA's West Side, Downtown, and Koreatown, as well as neighboring cities including Santa Monica, Burbank, West Hollywood, Torrance, etc. again, to do this, multiple inviduals or families simply pool resources and rent an apartment or house and share that space.

given that about 9% of Beverly Hills's population falls below the poverty line, it's a safe bet that this high density renting strategy is also employed within that city.

just from examining the nearby BH/LA border, we see that there may be other "invisible borders" (e.g. "Our prices discriminate so we don't have to") that prevent two populations from really fully merging.

it seems likely that any "open border" is actually accompanied by a unique set of additional, invisible borders, and cultural, financial and linguistic "force fields," sometimes powerful, sometimes weak.