After a slow start, the annual online auction for a steak lunch with billionaire investor Warren Buffett has drawn a $51,000 (U.S.) bid, a number sure to rise by the time the auction ends Friday night.

The winning bidder and up to seven others will dine with Mr. Buffett at the Smith & Wollensky steakhouse in New York.

Proceeds will benefit the Glide Foundation, a San Francisco non-profit that offers meals, health- and child-care, housing and job training for the poor and homeless.

Arguably the world’s most admired investor, Mr. Buffett drew no bids in the first 15 hours of the auction, which had a starting bid of $25,000.

By 4 p.m. ET, four bidders had made 18 bids, under names that eBay keeps confidential.

The top bidder appears to be a “proxy bidder.” Such bidders tell eBay in advance the most they will bid, and are awarded the top bid automatically if others bid meaningfully less.

Bids typically soar in the last two hours of the auction, which closes on Friday at 10:30 p.m. ET.

Last year Salida Capital Corp., a Toronto-based wealth management firm, won with a $1,680,300 bid. The record bid is $2,110,100 in 2008 by Hong Kong-based investor Zhao Danyang. The 10 prior auctions have raised more than $5.9-million.

Mr. Buffett began donating the lunches after his first wife, Susan, introduced him to Glide and the Rev. Cecil Williams, who founded the non-profit more than 45 years ago.

Susan Buffett died in 2004. Warren Buffett remarried in 2006.

Mr. Williams said the lunch covers about one-tenth of Glide’s $17-million annual budget, and is crucial this year because donations are down while demand for Glide’s services is up.

Mr. Buffett is worth $47-billion according to Forbes magazine. He built his fortune through his Omaha-based insurance and investment company, Berkshire Hathaway Inc. (BRK.A-N105,120.00170.000.16%)

In 2006, Mr. Buffett pledged most of his wealth to the Bill & Melinda Gates Foundation and four family charities.