In many ways, "Gift of the Magi" is a story about what it means for something to be valuable. Does something's value lie in how much money it is worth? Or are other things more valuable than money? The main characters are very poor – this is repeatedly emphasized – and yet the story suggests that their love for each other makes them very rich. It is that love, which motivates them to give up the only things of monetary (or personal) value they have to buy presents for each other. Perhaps their poverty is what enables them to appreciate what really matters.
Jim and Della's two prize possessions are valuable because they are the only things they have that are worth a significant amount of money.
Madame Sofronie understands the value of an object as the amount of money it will fetch.