Costly grace is the treasure hidden in the field; for the sake of it a man will gladly go and sell all that he has. It is the pearl of great price to buy which the merchant will sell all his goods. It is the kingly rule of Christ, for whose sake a man will pluck out the eye which causes him to stumble, it is the call of Jesus Christ at which the disciple leaves his nets and follows him.
Costly grace is the gospel which must be sought again and again, the gift which must be asked for, for the door at which a man must knock.
Such grace is costly because it calls us to follow, and it is grace because it costs a man his life, and it is grace because it condemns sin, and grace because it justifies the sinners. Above all, it is costly because it cost God the life of his Son: ye were bought at a price," and what 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 for us. Costly grace is the Incarnation of God.
Costly grace is the sanctuary of God; it has to be protected from the world, and not thrown to the dogs. It is therefore the living word, the Word of God, which he speaks as it pleases him. Costly grace confronts us as a gracious call to follow Jesus, it comes as a word of forgiveness to the broken spirit and the contrite heart. Grace is costly because it compels a man to submit to the yoke of Christ and follow him; it is grace because Jesus says: "My yoke is easy and my burden is light."
Deitrich Bonhoeffer, The Cost of Discipleship, pp. 47-48