I read somewhere a thread about jerry (in real life) liking porsche's and someone had said that in the show, jerry couldn't have afforded one because of the money stand up comics made.
I was watching the alternate side last night and when his rental car gets smashed up he absorbs the 2 grand cost of the repair. The other thing that comes to mind is he buys his father a cadilac, then rebuys it from jack klompus so just how much money does he (in the show) have??!!
It strikes me that he probably does earn a fair bit from his stand up shows in the series, but he isn't supposed to be as famous as he was in Real Life, (his fame pre-seinfeld would be closer the mark). We often hear refernces about him being on the road for 2 weeks, doing two shows in one night followed by the movies, (or at least it was planned that way), or i have 4 shows this weekend, suggesting he tries to work frequently, but in stints rather than every night.
Judging by the way he can get on the tonight show but David Letterman doesn't know his name, (before anyone points it out, i know it's a joke between friends), he looks like he is reasonably sucessfull, but more in the mould of the comedian you would see occasionally on a chat show, find funny, then 2 years later see him again thinking, "I know him, I've seen him somewhere before, he's funny, what's his name?" So the way he has lots of cash at some points and looks strapped at others doesn't surprise me, That's showbusiness when you start out.
Plus, there's all those $0.12 royalty checks from the Super Terrific Happy Hour. Even though the rain ruined the first batch, they just keep on comin'.