Rich Without a Ransom, by Alan Cupp
One point three million dollars. That's what 13-year-old Lucas Cage inherits from Mr. Casey, a neighbor Lucas thought had very little. In spite of his wealth, Mr. Casey's 84 years on earth were lived out rather modestly. Having no close friends or family, the old man left his fortune to Lucas, someone he considered a true friend.
According to the will, the money is to be put into a trust fund that Lucas won't be able to access until he's twenty. That's not a problem until Lucas is kidnapped and finds himself in the unusual circumstance of being rich without a ransom.