The most efficient way to make money is the basketball in the police station gym.
This is not what I have found.
I played basketball for 10 minutes, and made a total of $675. Granted that I don't have very good hand-eye coordination or timing, I suppose it could have been higher if I had.
I hunted treasure for 10 minutes, and made a total of $3779. Most of that time was spent in the area between the first forest area and the backyard of the church, which is presently the richest area, where you will never find stone figurines and where you can find silver figurines, but mostly jade figurines, clay idols and clay statues. There's a lot of luck involved, but it comes down to patience rather than hand-eye coordination or timing.