Dietrich Bonhoeffer in his book, The Cost of Discipleship wrote some of the most profound meditation on grace.
"Grace is costly because it calls us to follow, and it is grace because it calls us to follow Jesus Christ. It is costly because it costs a man his life, and it is grace because it gives a man the only true life. It is costly because it condemns sin, and grace because it justifies the sinner. Above all, it is costly because it cost God the life of his Son: 'Ye were bought at a price', and what has cost God much cannot be cheap for us. Above all, it is grace because God did not reckon his Son too dear a price to pay for our life, but delivered him up for us. Costly grace is the Incarnation of God.”
Grace is very expensive, so expensive that no one can afford to pay its price. It is so expensive that God had to establish a sort of Divinely designed welfare system for humanity to be able to attain it. This plan makes grace free to us, but, as it always is the case, someone else had to ultimately pay the price.
Jesus paid it. He paid it all. He paid it as he accepted the bitter kiss of Judas. He paid it as He allowed His torturers to unleash their evil-inspired hatred on His precious flesh with every lash. He paid it as He carried the heavy cross. He paid it on the road to Calvary. He paid it with each nail that pierced His skin. He paid it as He carried our sins.
It is, therefore, free to us, but highly costly to Christ.
And to think that He did all these for us…little, underserving, ill-natured us…
for all have sinned and fall short of the glory of God, being justified as a gift by His grace through the redemption which is in Christ Jesus; Romans 3: 23-24
It is costly because it cost God His Only Son.
It is costly because it cost the grieving of the Holy Spirit.
Grace is costly because it cost Jesus His life.