I have heard it said that we should not give what cost us nothing. The justification for this is 2 Sam 24:24
And the king said unto Araunah, Nay; but I will surely buy it of thee at a price: neither will I offer burnt offerings unto the LORD my God of that which doth cost me nothing. So David bought the threshingfloor and the oxen for fifty shekels of silver.
David’s giving here does not preclude him from giving that which he obtained for free.
While we should give that which costs us, we can also give that which is excess. An example of this would be the actions of the good Samaritan. He gave and the cost was minimal. It is not said whether or not he was giving because what he had obtained he obtained for free. The point is he gave.
Giving demands that we release the objects from the pleasure we gain from possession. This can be hard for some whether or not it cost them anything to obtain what is being asked.
This is not an argument for giving cheaply. It is simply a reminder that sacrifice always has a cost and that not all honored giving need be sacrificial since we can give according to what is in our hearts.
So let each one give as he purposes in his heart, not grudgingly or of necessity; for God loves a cheerful giver. (2 Cor 9:7)
Giving against the pleasure of possession is the higher law over cost.