What do Jim and Della value the most in the world? What do they value most in the world by the end of the story?
Jim and Della value non-material things more than anything else.
While the question seems to imply that what Jim and Della value at the beginning of the narrative differs from what they value in the end, there is evidence to the contrary. Granted, although Della is proudest of her luxurious hair, "shining like a cascade of brown water," and Jim is proud of his gold watch--
Had King Solomon been the janitor, with all his treasures piled up in the basement, Jim would have pulled out his watch every time he passed just to see him pluck his beard from envy--
nevertheless, the young couple obviously value most their love for each other. For, as narrator O. Henry mentions, despite having his salary cut, Jim is "greatly hugged" by his wife Della, who loves him as much now as she has before his wages were lowered.
Also, that Della would so quickly sacrifice her beautiful tresses, in which she takes a "mighty pride," in order to buy the watch fob for her beloved Jim is proof of her placing a higher value on love than on a physical possession. For, her greatest wish is to bring joy to her beloved husband.
Likewise, that Jim, who is so proud of his heirloom, the gold watch, would willingly sell it in order to give his wife the pleasure of possessing beautiful combs she has envied is clearly proof of his higher regard for her happiness and love than for material things.
Truly, then, Della and Jim Dillingham are a young couple who know what it means to love since they acknowledge the unselfishness of real love. Moreover, they are willing to deprive themselves of the pleasure of material objects in order to provide the other a gift of love. Indeed, they are, as O. Henry declares, "of all who give gifts...the wisest. They are the Magi."
At the beginning of the story, Jim and Della value things more than anything else. For Jim, his prized possession is the watch that he has inherited from his grandfather. For Della, it is her long beautiful hair that she prizes most.
Of course, in the middle of the story, they each go and get rid of their prized possession. They do this so they can each buy a present for the other.
By the end of the story, then, what they value most is each other and their love for each other. They have realized that these are much more important than things are.