How was George able to afford his apartment in NYC...
...if he was perennially underemployed? He's not good at his job and he never seems to be at work. He's always meeting Jerry at the coffee shop or his apt during the day. Plus, George's parents aren't rich.
Granted, George Costanza did move back in with his parents in Queens at one point. And the last several years, he had that office job at Yankee Stadium.