In today's money it's the equivalent of about $150,000, and funded by selling a plane that sold for twice that amount. I would classify having enough disposable income to own a $300,000 plane & a net worth of a few million to be wealthy.

I should note that my working definition of "wealthy" doesn't mean they never have to work again, or don't have to work to maintain their desired lifestyle. Other people may have different benchmarks. As you said, abstract measures aren't easily quantified, and someone else's benchmark for wealthy may be higher.