This is one of those places where the text is not specific. When I needed details, I supposed the 30,000 pounds 'came from her mother' - that is, the 30K was her mother's dowry, and when Lady Anne died, Mr Darcy (in a burst of sentiment) put it in his will to leave that amount to his daughter. I also decided Darcy had not yet considered the idea of his sister being married, but he had begun setting aside money to settle on her 'someday'. While I am dubious Wickham would actually know the income of Pemberley, in this construction he might have been privy to some details of the will, as he had a bequest from Mr Darcy himself. (I have always supposed 'the reading of the will' was a plot device for movies, but if not, he would certainly have heard of Georgiana's bequest in this version of fictional reality.)
Naturally it is just as easy to conclude Darcy is only 'sort of rich'; I'm not arguing for or against either interpretation, only saying I think there is enough ambiguity in the text to allow authors plenty of leeway on this point without actually departing from canon. And I wrote a story where Mr Bennet invented forced-air steel production and was fabulously wealthy, so I couldn't complain if someone made Fitzwilliam Darcy the first Batman.
Naturally it is just as easy to conclude Darcy is only 'sort of rich'; I'm not arguing for or against either interpretation, only saying I think there is enough ambiguity in the text to allow authors plenty of leeway on this point without actually departing from canon. And I wrote a story where Mr Bennet invented forced-air steel production and was fabulously wealthy, so I couldn't complain if someone made Fitzwilliam Darcy the first Batman.