They're not all upper class, Joey starts as a small background / ad actor, Rachel and Phoebe do small jobs like waitress and massages - Phoebe living in the street as a kid comes up a few times. (well, Rachel is born rich but she starts the series getting cut off and left with nothing - after she dumps her dad's credit cards she had) An episode has Joey, Phoebe, Rachel point out that they're poor and often out of a job so they complain that they aren't nearly as loaded as Chandler, Ross, Monica, and they have a hard time keeping up with their lifestyle. Also confirming Chandler, Ross, Monica are indeed well off. Later, Joey gets a good acting role, Rachel has a good job, and Phoebe gets a good place at some point.