Well, remember, Monica and Rachel's apartment belonged to Monica's grandmother. She mentions at one point that she's subletting the apartment and otherwise wouldn't be able to afford it. But this wasn't mentioned until later in the show I believe. The episode where it shows us how things came to be. Chandler finding a roommate, Joey and Monica having a crush on each other, Rachel engaged to marry Barry. etc...
So it doesn't sound like Monica would be able to afford that apartment if it weren't for her grandmother and all that. I don't really know who is responsible for payment if she passed away. But I like to think by the time Monica owns her own restaurant, Chandler moving in with his big business company salary, they could afford such a place. The Friends gang seems to become more and more successful in their careers allowing them to afford such places. However, at first I don't think they could afford it but again Monica was able to because of her grandmother.
As far as Chandler and Joey, I think Chandler could always afford his place and Joey just helped out when he could. lol. And I guess Ross and Phoebe did well enough at the start of the show to afford their places.
And I doubt based on searching the building based on exterior shots is an accurate guess to their monthly rent in the show. Even if it is, I bet in the story of Friends it could be something less. But that's usually not important. :)
"Emma! Why did you leave us?!"