John Lightfoot on Mark 12:41

[The people cast money.] They were casting in small money there. According to his pleasure, any one might cast into the chests how little soever he would; namely, in the chest which was for gold, as little gold as a grain of barley would weigh; and in the chest for frankincense, as much frankincense as weighed a grain of barley. But if he should say, Behold, I vow wood; he shall not offer less than two pieces of a cubit long, and breadth proportionable. Behold, I vow frankincense; he shall not offer less than a pugil of frankincense: that is, not less money than that which will buy so much.