How does the narrator underscore the value of these " treasures of the household"?
The phrase "treasures of the house" occurs in the last paragraph, so I am assuming that is what you are talking about.
I would say that the narrator underscores or emphasizes the value of the things that Jim and Della gave each other by putting them in the same paragraph with the wise men from the Bible.
In the Bible, the wise men brought very precious things -- gold, frankincense and myrrh. By mentioning the wise men right along with Jim and Della, the narrator is (in my opinion) saying that the gifts Jim and Della gave were just as valuable as the gifts the wise men gave Jesus.
I am not certain what you mean by treasures of the household. If you are referring to the gifts that each gives one another in the end, they are underscored because the true gift in the story "The Gift of the Magi" is the sacrifice and love that each one of the couple had made for the other.
There is no greater love than to give up that which is important to one. The husband gave up his watch to buy the wife the combs. The wife gave up her hair to give her husband the watch attachment. In the end when the gifts had been opened they become irrelevant because neither one can use them.
The husband laughs at the irony of the situation. He sees that their love is the true gift.