] In P&P, isn't 2000 a year what the Bennetts have? ...could just barely make it stretch.

From the start of Ch. 7 of P&P:
"Mr. Bennet's property consisted almost entirely in an estate of two thousand a year..."

However, I suppose it's a different case to have that income if your property is entailed (so no part of it could ever be sold, etc.) and you have five daughters who need dowries, for whom you haven't saved a penny. I think that 2000 a year is plenty to live on, but being entailed, none of it could be left to his daughters and their mother in a will and Mrs. Bennet's fortune of 4000 pounds divided among the five girls and herself would be all they would have after Mr. Bennet died.

Colonel Brandon, on the other hand, owns Delaford outright, and would be able to leave it all to Marianne and their children, even if they only have daughters. He also seems better able to manage that amount of money, since the estate was left to him in debt five years previously and he has made it profitable again.

] why the incomes mentioned seem so low compared to the incomes JA mentions.

I don't know anything about inflation rates, but maybe it was not as expensive to live in a relatively isolated part of the country, well north of London and the larger centres?