This is a difficult question with a complicated answer. On the surface, the bet was not worth the results for the banker because over the fifteen years his foolish choices had dwindled his massive fortune down to just about two million exactly so that if the young man claimed his winnings, the banker would be bankrupt and, worse yet, humiliated.
On the surface, the bet was not worth the result for the young man either because he is described in the most cadavarish terms when the banker approaches him to take his life. He is ill, weak, broken, and barely a man any more. Yet, when his letter is read, it seems that the young man gained more from his imprisonment than he could ever have to gained from a free and carefree life of happiness.
Unless the thoughts in his letter are the Freudian rationalizations of a distracted and fremzied mind, the young man may say that, in deed, the fifteen years of imprisoned seclusion have given him gifts more rare than all life, love and freedom could have ever done. As a corollary, the banker seems to have had an epiphany of sorts about his pampered extravagance and vain selfishness because of what the young man wrote of himself and his determination to break the bet so as to renounce the two million.
If the banker's epiphany is genuine and he will have an Ebenzer Scrooge type transformation, then the banker may say his side of the bet--which was relatively effortless, except for the heavy weight of immorality on his conscience--was worth the results.
The fact that the banker's last act in the story is to safeguard the letter of renunciation in a fireproof safe doesn't bode well for the banker's spiritual awakening going any deeper than relief at not going bankrupt and at not committing murder. Chekhov leaves just enough doubt for us to question the young man's sanity and the banker's sincerity, and therefore doubt the happy outcome of fifteen years of voluntarily imposed imprisonment. Despite the letter and the tears of cleansing, it still looks to the reader that the bet was not worth the results.
This is a great answer. I'm working on leading a discussion of this short story for a homeschool highschool lit co-op, and I believe this answer to be very true to the story. There is no "happy ending"; the banker's deceitfulness at the end shows that he is not yet a truly wise and good man, and the fact that the lawyer climbed over the gate and ran away shows that he, too, has not learned what it means to be fully human, in spite of his greater spiritual wisdom.
So, Chekov leaves us wondering... not a bad way to end a story.