They make ALOT for the time they work. Until you are "in", or should I say... a "made man"... you don't get many hours, hence poor pay. That said, the guys who work gravies make some serious premium cash, for not always doing all that much.
Other guys work their asses off doing good work, for standard pay.
You have to show up at the hall every morning, looking for whatever scraps are thrown your way. If you're high up enough on the union ladder, you get the good jobs...and then it goes down hill from there (hence my first comment).
Once you're in good, it's good money. Plain and simple.