Well he was a member of an old and noble (if a little out of the limelight) family with enough clients and enough of a network that he was worth something. The pirates knew he was worthwhile ransoming, and Caesar knew they knew that, so neither the pirates nor Caesar had any reason to contemplate the young man's death. In the end he was ransomed for a large amount so the pirates were right. The fact that Caesar went back and killed them later doesn't change the reality of the hostage situation at the time.