I worked with and know an ex pro. Asked him this exact question. For him, top 20 one year, there was a very small salary plus travel expenses, race fees, insurance, and bikes and parts. It was excepted that racers sell off kit at the end of the season to help fund a lifestyle. He said his best year, actually the year before he quit when he wasn’t really trying, his “wage” was about £10k. But he lived in his race van all season and he rode his bike for a job all over the world.
His parents are seriously well off and supported him through his pro years. From what he told me, the Athertons etc. are a business. It’s a full time job just managing the sponsorship and profile of the team to ensure the team are there to sponsor. If you get my drift.
He now has his own engineering business and tests for Specialized. Last time I saw him he was still sponsored by some parts, clothing, shoes and sunglasses. Not enough to make a fortune, but he swore loudly when he found out how much the rest of us have to pay for parts.