To clarify, the octopus mold itself isn’t particularly rare or expensive. The article refers to this individual piece as a “holy grail” because among the parts of the Cornwall flotsam, the “octopuses are considered the most prized finds as only 4,200 were onboard” in the lost cargo. The family in the article has been scavenging for years to collect the various parts, so this is something that is valuable to their subset of collectors but not really valuable to the typical Lego collector or fan.