According to Celebrity Net Worth, Philbin was making $20 million per year in salary "at his peak", but he wasn't taking home that amount from Who Wants to Be a Millionaire alone -- his other TV gigs also contributed to the money in the bank.
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How much Regis Philbin was paid to host Who Wants to be a Millionaire - ZarasLounge
How much Regis Philbin was paid to host Who Wants to be a Millionaire - ZarasLounge