The banker states to everyone at the party that he believes the death penalty is more humane and moral than life in prison.
"I have not tried either the death penalty or imprisonment for life, but if one may judge a priori, the death penalty is more moral and more humane than imprisonment for life. Capital punishment kills a man at once, but lifelong imprisonment kills him slowly. Which executioner is the more humane, he who kills you in a few minutes or he who drags the life out of you in the course of many years?"
Asking about what he values most in life based on that quote is an interesting question, and readers have to infer through this indirect characterization what the banker might value most. I think that it could be argued that the banker values most a hedonistic life style. He lives for pleasure, and if he is not having any fun, then he isn't accomplishing what he was put on Earth to do. To the banker, life without fun isn't worth living. That's why he believes an immediate execution is preferable to life in prison. Life with suffering isn't worth living. Readers get a bit of support to this character trait moments after he and the lawyer agree to the bet.
The banker, spoilt and frivolous, with millions beyond his reckoning, was delighted at the bet.
The banker has more money than he knows what to do with. The money being put on the line is a trivial amount to him, and it is entirely worth the money because the bet is simply something fun for him to spend his money on. Putting a man in prison for 15 years doesn't concern him as long as the banker is having fun with the bet.