I wouldn't set it at $500M. I would set it at 1000x the median household income. Once you've hoarded enough wealth to last ten long lifetimes, we're cutting you off. This would be about $80M in today's money.

This is also roughly my best estimate for the largest possible "honest fortune," one that can be obtained through wage labor and investment. This is the kind of money a married pair of neurosurgeons could amass if they both worked long careers, never had kids, lived like monks, and invested everything. To get more money than this, you have to own a company, be an executive, or otherwise exploit the labor of others.