A ten-minute piece on Motivation, by Dan Pink, author of Drive (with… - Zer Netmouse — LiveJournal
$1M would be the lower bound; it assumes about a 3% inflation-adjusted ROI, to supply $30k/year in living expenses. That may be a little aggressive, but I don't think it's unreasonable for a single person over a long timeframe. I'm also not counting the cost of a primary residence in the $1M, and a person with a chronic health condition would definitely not be able to make it on that much.
That said, I initially arrived at these numbers in 2000, so to properly inflation-adjust them, they should probably be increased by about 20%.