This shows us something of the intensity of Mary's love for Jesus. She must have known what she was doing, and how much it had cost her. But she did not care. Her love for Jesus was so great that she was not concerned about the cost. If she had sorrow, it was not about how much she was giving up, but that she had so little to give.
Love often expresses itself in self-sacrifice, with little thought for self.
If an offering is to be meaningful, it should cost us something, and it should be done out of our own free will. We should give up something that is of value to us. Maybe it requires everything we have, and everything we are. The disciples were concerned with self. They wanted to be great in the kingdom of God.