I believe the Tiebout Hypothesis applies too. Research that applies to the US, and tho' somewhat dated, probably still quite relevant though.
Point is - people would move interstate IF they perceived that that move would bring them about a 20% increase in earnings. Though the modern immigrants may not be all that great on their own particular individual mathematical circumstances. Many may not be able to count at all.
Yet as a group the concept applies - & I suppose for an international move that factor may be a bit higher.
Importantly, its a perception of difference, and not necessarily a real difference. So while people think they can do better across the Rio Grande, Atlantic, English Channel, Mediterranean, Bosphorus, Tasman Sea & the Sea of Arafura - they will keep coming.
While people think in their slums across the world, that the streets of Melbourne & Sydney are paved with gold, they will continue to make their way towards these cities.
