According to Official Guinness Records,
The most valuable piece of bread is a grilled toasted sandwich which sold for $28,000 (then £15,000) on Ebay in November 2004. The original owner, Diane Duyser of Miami, USA, was just about to tuck into her sandwich when she noticed it contained an image of the Virgin Mary. She sealed it, kept it for 10 years as a lucky charm, then went onto sell it to Goldenpalace.com, a Canadian online casino.
For a complete list of 2003 records, please visit 2003 Guinness Records in United States.