What is the moral of "The Bet"?

One of the morals of the story "The Bet" concerns a lesson on human folly and pride. Folly is when someone makes foolish choices. In this story, the characters of the banker and the lawyer are so prideful and arrogant that, to prove their individual points about solitary confinement, they sacrifice important things in their life. Because the banker felt angry that his views on death being preferable to a life of isolation were being challenged, he risked two million rubles to prove his point. Because the lawyer wanted to prove his point that any life is better then death, he sacrificed fifteen years of his life in isolation.
In the end, the results of their prideful and foolish bet have terrible consequences. The lawyer loses his desire to live on earth and comes to hate the normal pleasures of human existence. He says in a letter to the lawyer:

I despise your books, despise all worldly blessings and wisdom. Everything is void, frail, visionary, and delusive as a mirage.

The lawyer says in his letter to the banker that he will give up the money he would have earned on winning the bet and leave five minutes before the bet would have been completed. This allows the banker to keep his money; however, in the banker loses his own self-respect in the process. He realizes how foolish he was to make such a bet just to bolster his pride. He knows that he has ruined a man's earthly happiness forever, and he despises himself for it. The story says that never before had the banker "felt such contempt for himself as now."
In short, this story warns the reader of the dangers of impulsiveness and pride. The thoughtless choices individuals make in the heat of emotion can often have devastating, life-changing effects.

I would argue that the moral of the story is that the material world and everything in it is ultimately of no importance. Although the lawyer turns his back on all the knowledge he's acquired during fifteen years of solitary confinement, at least he's learned something from the experience. Which is more than you can say of the banker, who still remains trapped in the world of material objects, every bit as confined in it as the lawyer was in the banker's lodge during his lengthy period of self-imposed isolation.
Whatever happens from now on, we can be fairly sure that the lawyer is more spiritually free than the banker, who seems not to have learned any lessons from this unusual wager. Even if the lawyer should die the very next day—and given his fraught mental and physical state, that's more than a distinct possibility—he will do so in the assurance that he's freed himself from the constraints imposed upon him by a world in which material objects count for more than spiritual values.

The story is told entirely from the Banker's point of view, although it is the transformation of the Lawyer that contains the real message or moral. We only know what is happening with the prisoner because of the things he orders from his keeper. The prisoner naturally reads many books. He even learns foreign languages so that he can read other books. Over the long, solitary years his reading becomes more and more serious, judging from the titles and authors he requests. But then:
Then after the tenth year, the prisoner sat immovably at the table and read nothing but the Gospel. It seemed strange to the banker that a man who in four years had mastered six hundred learned volumes should waste nearly a year over one thin book easy of comprehension. Theology and histories of religion followed the Gospels.
The prisoner will spend the remaining five years of his confinement studying and meditating on the Christian message contained in the four Gospels: Matthew, Mark, Luke, and John. He has finally, after ten years of solitary confinement, found the answer he has been looking for, and he seems finally content. He is evidently not interested in the teachings and rituals of the Russian Orthodox Church but in the original words of Jesus Christ himself. Apparently Anton Chekhov felt the same way, as did his friend Leo Tolstoy.
The most pertinent verses in the Gospels, as far as this story applies to both the Lawyer and the Banker, are probably to be found in Chapter 6 of Matthew, which are rendered in beautiful English in the King James translation of the Bible.
Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal.
But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal.
For where your treasure is, there will your heart be also.
Thoughts such as these are what inspire the prisoner to relinquish the two-million rubles which the Banker would be forced to pay him next morning if he stayed. Both the Banker and the Lawyer realize the truth of what is expressed in Matthew 6.