i have friends who do it. some weeks they can make $500, other weeks not so much. it all depends on the day and the workload, and who else is working.
to pull $500 in a week is hard. messengering is hard and dangerous work, and you get little respect.
fax machines by no means killed the messenger industry, granted, it's not a big as it used to be, but it's still there.
there's no set salary for it.
one of my friends who does it has a schwinn madison track bike, a harry havnoonian track bike (both with phil wood wheels) a carter track with full campy pista, two merlins, one with full 03 record, and i have no clue what's on the other one, his "winter beater bike," and a couple high-end BMX bikes, which are not cheap, BTW.
he doesn't have a family or anything, but he lives in a pretty decent neighborhood. but he works his ass off.
he also hates his life most of the time beacuse of work. his shoulders are a mess, and he's been hit about 7 times in the last three years...
then again, i know a lot of other people who do it and they love their jobs, and they're pretty much in the same situations as he is.
i've thought about doing it, but i never had the time when i lived in boston, and now that i live in berkeley, i don't know SF well enough that i'd feel confident doing it, and SF is way more dangerous than boston.