Lair of the White Worm: Chapter XIII
As proof that the book's Lady Arabella March is a mortal, unlike the films's Lady Sylvia Marsh, Lady Arabella is worried about money and her debts.
Of course, that is not to say that Lady Sylvia has no need of money. "You ought to know by now, that I shed my cars as frequently as a snake sheds its skin."
Which raises the question, how does an immortal get the money that they need. In other stories, the immortal has used their knowledge of their past to get a job in the present. I can imagine they can also bring forward items from the past that have little or no value in the past, but have value in today's market, such as coins, stamps, etc.