Depends on the girl and her situation. I think if she's got a degree and works w/ it, I genuinely don't see why I'd have to pay for than 5k. 10k at absolute most if she really cares what her family thinks.
Truth is this will never be a problem. If the girl likes you she'll accept any figure as long as it isn't disrespectfully low given your circumstances. Even a few hundred or less will be fine if you're broke students, just stepped out of HMP so and so, etc.
Nobody had ever not gotten married because they couldn't make mehr. If she sets an intentionally high figure, take it as a polite rejection.