LOS ANGELES—During a Keep A Child Alive charity auction last Friday, U2 frontman Bono paid $575,000 for a guitar signed by the Irish recording artist, outbidding his nearest competitor by nearly $500,000.

"Not only does this allow me to donate to a cause in which I very deeply believe, but I now own a unique and valuable piece of rock and roll history," said the singer, who also placed the winning bid on a "once-in-a-lifetime" lunch with Bono at $1 million.

Bono reportedly listed the autographed guitar on eBay the next day to raise money for Amnesty International, and within 20 minutes, had placed the highest bid at $750,000.