in order to benefit from that income if he wasn't living in the house and managing the estate directly, as Mr Bennet seems to do, Mr Collins would have had to put in a steward, which would cost him much more than putting in a curate at Hunsford. Mrs Bennet is certainly not capable of managing an estate, and her only capable daughters, if not married, would have to become governesses, companions or teachers because their mother would hardly be able to keeep them in food and clothes. Then there are the servants. Mrs Bennet would not be able to afford to staff Longbourn at the same level as when Mr Bennet was alive, but without their regular maintenance the house would begin to deteriorate, possibly letting Mr Collins in for expensive repair work when he finally took possession. Meantime he and Charlotte and possibly a growing family would be in a rather small parsonage. For all this he would (as Adrian rightly said) get a rent well below the going rate. Why would he enter into an agreement of so little advantage to him?