Della displays several basic human values through her actions in The Gift of the Magi.
She loves Jim. She works hard to preserve his dignity, even when their poverty makes it difficult to uphold the pretense that they are a fine couple in respectable society. She wants him to have the best of all things and hurts for him when this is not possible.
Della's love motivates her to undertake an act of great self-sacrifice. In order to give Jim the watch chain that would appropriately show off his treasured pocket watch, she gives up the most valuable possession she has - her hair. There is hesitation and regret and doubt, but all those feelings are overcome by her desire to give a worthy present, something that would come close to indicating the depth of her love for him.
In the end, Della's story illustrates that human value is not always based upon material wealth. While she and Jim had very little in terms of monetary property, they had everything they needed in the world - they had each other.