What does An-Mei mean when she says that her mother's pain is worth something?

What An-Mei means by this is that her mother has taken the pain she's endured through cutting herself and put it to good use, namely providing some much-needed succor to Popo in her hour of need. Even at such a young age, An-Mei recognizes this as an act of sacrifice. Out of the evil of her mother's pain has come the good of relieving Popo's suffering.
In ministering to Popo this way, An-Mei's mother is also transforming the emotional pain she's suffered for many years as a family outcast into something positive. It would've been all too easy for An-Mei's mother to have felt bitterness and rage at how Popo had treated her for so long. Instead, she's dug deep inside herself, both literally and metaphorically, drawing upon her physical and emotional pain in a selfless act of giving.

As a child, An-Mei lived with her grandmother, Popo. An-Mei's mother was not welcome in the home because she had married a man who already had other wives. An-Mei's mother had tried before to retrieve An-Mei when An-Mei was young, but got into an argument with Popo. During the argument, hot soup was knocked over and spilled onto An-Mei's neck, restricting her breathing. An-Mei miraculously survived, but was left with a scar. Later, when Popo became ill, An-Mei's mother returned again to care for Popo. Popo, weakened by her illness, allowed An-Mei's mother to tend to her. An-Mei's mother then cut a bit of flesh from her arm and fed it to Popo to help her get better. An-Mei witnesses this, and years later recollects, "Even though I was young, I could see the pain of the flesh and the worth of the pain."