Della wants to make this Christmas a very special one for Jim, but her savings is a paltry sum of one dollar and eighty-seven cents before the day of Christmas. This is what upsets her at the beginning of the story.
One dollar and eighty-seven cents. That was all.
Though small in value, it has taken her months to save that. In fact, she has been planning to get “something nice for him” for several months. How painstakingly she has been saving each cent is explained in the following line:
She had put it aside, one cent and then another and then another, in her careful buying of meat and other food.
Della does all she can to save as much as possible from her essential domestic expenses. The day before Christmas she counts her savings three times. We feel really bad for her. We can imagine how much she must have wished to have counted it wrong the first and the second times. She had hoped it might be more than one dollar and eighty-seven cents.
Depressed and helpless, Della knows it very well that what she’s got is an significant amount, certainly not enough to buy “something almost worth the honor of belonging to Jim.” This is why she is upset.