In 2023 his publisher said that his printed collections had sold 50 million copies worldwide, and that the strip had appeared in 2,400 newspapers. That's at least tens of millions of dollars over many years, and with little spending and risk-averse investments, it's not unreasonable to conclude $100M for the total net worth figure.
Yes, but what fraction of that money ever reached him?
I think a hundred million is probably a bit optimistic.
That said, I would be surprised if he's not at least a multi-millionaire.