In Amazon MSK, the pricing documentation has mentioned that - You pay for the amount of storage you provision in your cluster.
But while creating the msk cluster, we have defined initial volume size per broker is 100gb, And our cluster has utilized only 50gb during a month. So I have to pay for 100gb as I have defined, or I have to pay for 50gb used by cluster
You pay for the amount of storage you provision, not the amount of storage you use. As is clearly stated in the pricing documentation.
You provisioned 100gb of storage, so you will pay for 100gb of storage.