O. Henry's classic story "The Gift of the Magi" is about love and giving. Since the story is told from the point of view of Della, the reader knows more about what Della went through to give her perfect gift to her husband Jim. It is important to look at both gifts. 

Della valued her hair. It was her "crowning glory."  Jim  loved her hair. 

So now Della's beautiful hair fell about her, rippling and shining like a cascade of brown waters. It reached below her knee and made itself almost a garment for her.

On this Christmas Eve, Della had something more important than her hair on her mind: she needed money to buy her Jim a Christmas present. She would give anything for Jim, so she sold her hair.  She was worried about what he would say, but still, it enabled her to buy the watch fob that would hold his precious watch. 

The reader does not know much about the circumstances of Jim's present. 

Now, there were two possessions of the James Dillingham Youngs in which they both took a mighty pride. One was Jim's gold watch that had been his father's and his grandfather's.

This was a family heirloom. Jim's grandfather had bought it and used it; then, the grandfather had given it to his son, who used it; then, the father had given it to Jim.  It was a tradition that Jim hoped he would be able to carry on with his son. 

Della meant more to him then the watch.  He willingly gave it up to buy his Della a beautiful set of hair combs to hold her long hair in place. 

Each of the couple has given the most valuable thing that he/she possessed.  Della's hair was a part of person.  However, no less important was the personal loss of Jim's heritage. Della hair would grow back eventually.  What about Jim's watch? Would he be able to buy back his watch? This was a little more questionable. 

The most important part of the gift exchange was that the seemingly worthless gifts that they gave each other were so valuable after all.  As long as they have the gifts, they will each remember the selfless act that each performed to give the perfect gift to the other one.  That feeling cannot be bought, and its value cannot be estimated.    

Before the gift exchange, each person had a prized possession.  The watch was Jim's and the hair was Della's.  Both of these were sacrificed.  In the sacrifice, each of them gained something new.  The love that they share knowing that the other gave his most important possession for the other is more important that anything else.  In the gift exchange, the couple is now closer together based on the love that was required to give the gift. 

Who sacrificed more? Possibly Jim--he may not be able to buy his watch back.  But this is not the important part of the story.  It is what each received from the other that makes this a grand love story.