Colloquially, expat tends to imply self funded, or at least bringing wealth with them in some respect. That’s not what the word literally means but it’s how I see it used. This is consistent even with foreign professionals coming to work in, say, London. Few people refer to them as immigrants though that’s what they are technically. (I’ve seen people be called an “ex pat from their country” or an “international worker”, these both in the city of London referring to office based professionals) The word immigrant seems to be reserved (at least in Britain) as a pejorative because it implies low skill and by further implication, a net cost on public services.