In chapter 23 of Mansfield Park (or chapter 5 of volume 2), Edmund's expected living from Thornton Lacey is discussed by Mary and Henry, as they observe Edmund talking to Dr. Grant. The Thornton Lacey living is less than half of what could be gotten from the Mansfield living currently held by Dr. Grant.
"... Dr. Grant is giving Bertram instructions about the living he is to step into so soon. I find he takes orders in a few weeks. They were at it in the dining-parlour. I am glad to hear Bertram will be so well off. He will have a very pretty income to make ducks and drakes with, and earned without much trouble. I apprehend
he will not have less than seven hundred a year. Seven hundred a year is a fine thing for a younger brother; and as of course he will still live at home, it will be all for his menus plaisirs; and a sermon at Christmas and Easter, I suppose, will be the sum total of sacrifice."
Mary has a fortune of 20,000 pounds and her brother's annual income is four thousand pounds. So, she is not impressed by Edmund's imminent income.
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"Nothing amuses me more than the easy manner with which everybody settles the abundance of those who have a great deal less than themselves. You would look rather blank, Henry, if your menus plaisirs were to be limited to seven hundred a year."
"Perhaps I might; but all that you know is entirely comparative. Birthright and habit must settle the business. Bertram is certainly well off for a cadet of even a baronet's family. By the time he is four or five and twenty he will have seven hundred a year, and nothing to do for it."
It is not just the money for Mary. She shares the contempt her brother show for the gentlemanly profession of the clergy ("nothing to do"). Indeed, if she did marry Edmund, they would hardly be poor given his income and her fortune.