Why is Helen Stoner concerned that Sherlock Holmes won't take her case?

Helen Stoner is afraid Sherlock Holmes will refuse to help her because she does not have enough money to pay his fee. She tells him:

Oh, sir, do you not think that you could help me, too, and at least throw a little light through the dense darkness which surrounds me? At present it is out of my power to reward you for your services, but in a month or six weeks I shall be married, with the control of my own income, and then at least you shall not find me ungrateful.”

Her stepfather Dr. Roylott controls her capital and her income, but according to the terms of her deceased mother's will, Roylott will have to pay her one-third of the income from her mother's estate every year when she gets married. Holmes will later do some research and learn that this will amount to about 250 pounds a year. And it turns out that Roylott is trying to murder Helen in order to avoid having to pay her that sum of money. Holmes establishes that Roylott murdered Helen's sister Julia two years earlier for the same motive: Julia was engaged to be married and would have been legally entitled to a payment of about 250 pounds a year. Roylott has gotten himself into financial difficulties and would be virtually destitute if he had to pay either of his stepdaughters that much money out of his dwindling income.

But Holmes takes Helen's case out of sympathy for the terrified girl. He is also intrigued by the unusual nature of the case. The story is what is called a "locked room murder mystery." Assuming that Helen's sister Julia was murdered, how could the perpetrator have managed it when the girl was sleeping in a room with the door locked and the window-shutters tightly closed and bolted? 

Sir Arthur Conan Doyle wrote four Sherlock Holmes novels and fifty-six Sherlock Holmes short stories. He established that his detective would take cases on a pro bono basis because this would enable the author to deal with a wider variety of characters, plots, and settings than would have been the case if Holmes only worked for the affluent class of people who could afford to pay his fee. It enabled Conan Doyle to continue to be creative and extremely popular on both sides of the Atlantic. This unusual altruistic attitude of the great detective can be observed in two of the most popular Sherlock Holmes stories: "The Adventure of the Speckled Band" and "The Red-Headed League." Dr. Watson, who is the narrator of most of the stories, frequently explains that Holmes only takes cases that interest him. The detective has become so sought after that he no longer needs to be concerned about money. In one story, "The Adventure of the Priory School," Holmes' client, a wealthy English nobleman, writes him a check for six thousand pounds, which would be equivalent in purchasing power to millions of today's American dollars. Early in "The Adventure of the Speckled Band" Dr. Watson states:

...working as he did rather for the love of his art than for the acquirement of wealth, he refused to associate himself with any investigation which did not tend towards the unusual, and even the fantastic.