Omaha – Billionaire Warren Buffett has raised more than $20m for a San Francisco homeless charity since 2000, and he's about to add to that total by auctioning off a private lunch.The weeklong eBay auction that began on Sunday will wrap up Friday night. Six of the past eight winners paid more than $2m to dine with Buffett, the investor who leads the Berkshire Hathaway conglomerate, with the record so far coming in 2012, when the winning bid was $3 456 789.That 2012 lunch remains the most expensive individual charity item ever sold on eBay.The lunch auctions began after Buffett's first wife, Susie, showed him the Glide Foundation, where she had been volunteering. Susie Buffett died in 2004, but the connection between Warren Buffett and Glide's founders has endured.Buffett has praised the foundation's efficient and effect approach in providing meals, health care, job training, rehabilitation and housing support to the poor and homeless.'Astonishing'"I am proud to be part of something that has directly benefited so many people in need," Buffett said ahead of the auction. "Glide is a bridge for thousands of people on the brink of despair, helping them achieve dignity and opportunity by providing them with basic services."Rev Cecil Williams, one of Glide's co-founders, said the cumulative effect of the donations has been "astonishing."Buffett has said he gets a wide range of questions at the lunches that usually run for several hours. The only limit on lunch conversation is what Buffett might invest in next, but any other topic is open.The winners of the lunch auction typically dine with Buffett at Smith and Wollensky steak house in New York City, which donates at least $10 000 to Glide each year to host the lunch. But when the winner wants to remain anonymous, the lunch happens elsewhere.Buffett's company owns more than 90 subsidiaries including insurance, furniture, railroad, jewellery and candy companies, restaurants and natural gas and corporate jet firms, and has major investments in such companies as Coca-Cola Co, IBM and Wells Fargo & Co.