Just watched an old episode of BBT on syndication, and it appears Leonard drives a BMW. So even though we still don't know how much money they make, he's probably doing alright for himself.
It might be the Friends
-effect (no matter what job you have, you can afford to live in NYC in a nice apartment, etc.). However, they do have plenty of money to drop on comics, statues, collectibles, etc. Sheldon, especially, seems to have plenty of money. The time he lent money to Penny, he truly didn't care about when she paid him back. Sheldon's too uptight to just let something like that go, unless he truly didn't need or even want the money.
Sheldon: I see no large upcoming expenditures, unless they develop an affordable technology to fuse my skeleton with Adamantium like Wolverine.
Penny: Are they working on that?
Sheldon: I sincerely hope so.
The end of the episode showed that the others knew about where Sheldon kept his extra money and would take it, presumably without Sheldon caring.
Sheldon: In case either of you have larceny in your heart, you should know that I have moved my money out of the snake can.
Leonard: But if you're ever short, there's always a couple of fifties in Green Lantern's ass.
I just watched the first Wil Wheaton episode again. There was this exchange in that episode between Raj and Sheldon:
Sheldon: I’m sorry, Raj, but I have no interest in playing a game in which I find no challenge.
Raj: What about the money?
Sheldon: I have money.
Raj: But this is other money.
Sheldon: How does it different from the money I have?
Raj: Half of it will be mine.
Sheldon: Do you need it to buy a less disturbing sweater-vest?