Putting silver coins under the mast has a long tradition perhaps going back to roman times. In those days the coins may have symbolized a sacrifice to the gods or as payment to the ferryman that carries the dead souls to the afterlife. The practice of putting coins under the mast may have come from the practice of putting coins on the eyes of the dead. In today's modern aircraft carriers, the top officers of the new ship place thier silver aviator wings under the foundation of the control tower. This is a symbol of their sacrifice, giving up their careers as aviators to command a ship.
You can purchase a brand new silver dollar from the US mint for about $37.00.