Interesting, I hadn't thought much about the money before. Because of my job, I do happen to know how much scientists working at a university make, and it is no small amount. Based on what we've learned in the show, I would say Leonard is making $80,000 a year at the very least; based on the awards and reputation of Sheldon, I'd say he makes considerably more. Physicists are well paid (this of course has nothing to do with why I'm dating one
). Penny obviously makes much less than this, probably around $25,000 a year. That's a little over $2,000 a month, which realistically probably isn't enough to afford the rent/bills/etc. for the apartment she lives in. If you think of her apartment as a one bedroom its a little more believable. The bigger question, though, is why Leonard and Sheldon have decided to live in the building when they could afford to buy homes or at least live separately (or share a bigger apartment). It must have to do with reasons other than money. Maybe they've mentioned it on the show and I don't remember. Even with all their spending on comics, I'm sure they both have a lot of money saved up.
Of course, it's TV and it isn't supposed to be realistic, so none of this really matters. But still interesting to think about.