It isn't a family of 4, it's 4 women plus 3 servants. The amount of food required would be 75% greater than you are thinking. And there is clothing for 7, not just 4, though the servants might not need/receive new clothing as often as the family members. The servants also get wages, albeit much smaller than they would get today, so I don't know that I would call 500 pounds per year obviously adequate. As Barbara said, a more correct conversion might only result in an income of 14,000 pounds per year in today's terms, so that would certainly not be very comfortable for 7 people to live on.

It's very difficult to compare eras that are 200 years apart. At the very least, the Dashwood women will be stuck at home unless someone else offers them a ride or they are able to pay for transport -- unlike people today who have some forms of public transportation even if they do not have cars of their own.