John Calvin Commentary Luke 2:24

"and to offer a sacrifice according to that which is said in the law of the Lord, A pair of turtledoves, or two young pigeons." — Luke 2:24 (ASV)

And that they might offer a sacrifice, this sacrifice belonged to the ceremony of purification, so that no one would suppose that it was offered for the sake of redeeming the first-born. When the Evangelist mentions a pair of turtle-doves, or two young pigeons, he takes for granted that his readers will understand that Joseph and Mary were in such deep poverty as not to be able to offer a lamb. For this exception is expressly mentioned:

If she be not able to bring a lamb, then she shall bring two turtles, or two young pigeons (Leviticus 12:8).

Is it objected that the Magi had very recently supplied them with a sufficiency of gold to make the purchase? I reply: We must not imagine that they had such abundance of gold as to raise them suddenly from poverty to wealth. We do not read that their camels were laden with gold. It is more probable that it was some small present, which they had brought solely as a mark of respect.

The law did not rigorously enjoin that the poor should spend their substance on a sacrifice, but drew a line of distinction between them and the rich as to the kind of sacrifices, and thus relieved them from burdensome expense.

There would be no impropriety in saying that Joseph and Mary gave as much as their circumstances allowed, though they reserved a little money to defray the expenses of their journey and of their household.