Mrs Bennet's fortune, had her two eldest not married so well, would be £5,000, which Mr Collins helpfully tells us is invested in the 4%s. It would therefore yield an annual income of £200. Even if Mrs Bennet were somehow able to devote all of this to the rent (which she would not be, as she needs clothes and food) even a much more altruistic person than Mr Collins would be likely to balk at taking a rent of one tenth the income he could derive from the estate, to the disadvantage of himself and own wife and children. He would never do it, and I don't really blame him. It's not actually his fault that the entail exists, and in his mind he did make a generous offer to marry one of his nearly portionless cousins and she spurned it.