No, lord Tywin Lannister suggested that the mines at Casterly Rock had run dry, not the westerlands in general. And that was in the show only, in the books they just play themselves off as richer than they are. The Crakehall's not only took any Lannister mines and other lands, but after the beggars rebellion they seized the assets of what amounts to two other houses as well. They really ARE rich because of this combination of assets, so they do have more money than most other houses I believe, save perhaps the Tyrell's who may equal or somewhat hold slightly less, I'm not sure