In his book An Exposition of the Whole Bible, G. Campbell Morgan writes on Jesus and the poor widow in Lk 21.1-4:
Of the gifts being cast into the treasury He was the true Appraiser. He saw the widow as she cast in her gift, and said that she had "cast in more than they all." In the realm of superfluity God does not begin to count. The first entry in the heavenly books is that of sacrifice.
Well said.