Here's a link to a thread with another discussion. One post on the thread links to the Colquhon table, which is a comprehensive description of income distribution in England c. 1803.

Colquhon splits up some groups we would lump together and lumps some we would split up. For example, he treats baronets, knights, esquires and gentlemen without title as separate income categories; we would regard them all as non-noble landowners, whose income wouldn't vary by title. On the other hand, he lumps together as "persons of the law" at one extreme judges, who would earn several thousand pounds a year, and attorneys' clerks, who might earn 100 or so, at the other.

But if you compare Darcy's £10,000 per year or Mr. Bennet's £2,000 with what clergymen, "eminent merchants and bankers," shopkeepers or farmers made, you'll get a sense of where they stood economically.

And yes, Darcy's income would be pretty steady. It would come primarily from the rents of his tenant farmers, who were on long term leases, with some from government bonds ("the Funds"), which paid fixed interest rates.

Rents and interest were due on the four quarter days: Lady Day (Incarnation) (March 25), Midsummer Day (St. John the Baptist (June 24), Michaelmas (September 29) and Christmas. Scotland had different quarter days: Candlemas (February 2), Whitsunday (May 15), Lammas (August 1), and Martinmas (November 11).