in my limited reading I've seen two arguments about immigrants - the one you made (i.e., the ones most likely to come are most affluent b/c they can afford to move etc.), and an argument where the most likely to emigrate are poorer people seeking a better way of life. even really poor people from latin america, india, etc. can usually (after years of hard work) save up enough for a flight over. i think the first reason is most likely when the person emigrates for political reasons, the second most likely when the person emigrates for economic reasons. for my own family of immigrants, they left cuba for political reasons and they were solidly working class (grandfather was a butcher after having served in the cuban marines during WW2, dad had some mid-level government job), not rich by any means, but still better off than poor farmers, etc.
