Dietrich Bonhoeffer was a German PH.D. in theology, but he developed his view of discipleship and costly grace from a visit to an African American Church in New York City where he saw biblical discipleship really being lived out. He saw people who advocated for the least of these, who were busy serving every day of the week.
He took that experience with him back to Germany and used it to fight Nazism. It became the core of his view of Christianity that we are to be disciples of Christ in the same way the original 12 were disciples, and pay a similar price.
And Christianity comes with a high cost, even though salvation is free.