Kit, this question comes up all the time. Rather than considering what Darcy's £10,000 or the income from Wentworth's £25,000 would be in today's money, I think one gets a better sense of characters' relative social position by comparing their incomes with those of other people in the same era.

The Colquohon table, which was compiled in 1803, gives family incomes for various ranks, along with the number of families in each. Here's a link to a thread that includes Colquohon, plus some other occupational income data.

From Colquohon one sees that Colonel Brandon's or Mr. Bennet's £2,000 a year put them quite solidly within the ranks of non-titled landowning gentlemen, and the income from Miss Morton's £30,000, if invested in government bonds yielding between 3 and 5 percent, would put her above the average of "gentlemen and ladies living on incomes." Darcy is indeed "as rich as a lord," since Colquohon gives an average of £8,000 per year for titled landowners. A "lesser merchant" like Mr. Gardiner might make somewhat less than £1,000 per year, and a country attorney like Mrs. Bennet's father rather less. Artisans lived on £50-60 per year or so, and labouring people on about £40.

It's also worth looking at relative standards of living. The Dashwoods get along in Barton Cottage with one male and one female servant. The Bennets have a cook (Mrs B is offended when Rev. Collins asks which of the daughters is responsible for the meal he's just eaten) and can keep a carriage and horses (but the same animal doubles as carriage and saddle horse). Even the Bates household, living on thinly disguised charity, have a servant.

Whether one needs to bump up Colquohon for inflation during the later part of the Napoleonic Wars depends on the date at which P&P or S&S took place, which is an interesting problem in itself. Persuasion can at least be exactly dated to Napoleon's brief exile to Elba in 1814-15.