What quote gives the best example of the characterization of Jim's feelings for Della in "The Gift of the Magi" by O. Henry?

Jim's love for Della is evidenced by his decision to sell his most prized possession, his watch, to buy her a hair set. Jim loves Della's hair, and he knows she loves her hair, too. He is trying to do something nice and thoughtful for her.
This is why Jim is so stupefied when he arrives to find Della cut all her hair off. It is not because he thinks she is ugly without her hair, as she feared. Jim just spent a lot of time imagining how much Della will like her gift. He tries to show her how much he cares as soon as he comes out of his trance.

He enfolded his Della. For ten seconds let us regard with discreet scrutiny some inconsequential object in the other direction. Eight dollars a week or a million a year—what is the difference? A mathematician or a wit would give you the wrong answer.

Jim tells Della nothing she could do would make him love her any less. He gives her the gift by way of explanation for his reaction to her hair. He appreciates the sacrifice she made for him, and when he sees his gift, his response is both touching and practical.

"Dell," said he, "let's put our Christmas presents away and keep 'em a while. They're too nice to use just at present. I sold the watch to get the money to buy your combs. And now suppose you put the chops on."

Jim understands the best gift Della could give him is a demonstration of her devotion and love for him. Cutting her hair was a greater gift than the watch chain because it was a sacrifice. He made a sacrifice for her, and she made a sacrifice for him. It proves they love each other.