Cloud migration cost can be categorized based on three groups: Direct (TCO) cost, Indirect cost and Migration cost itself.
Direct cost can be calculated relatively accurately with various Cloud calculators (e.g. AWS
An example of the indirect cost might be the case when business lead puts a lot of efforts towards the migration and doesn't pay enough attention to the marketing which impacts the revenue
Cost of the migration includes, but is not limited to, data transfer from on-premises and to the cloud and the cost of expertise required to perform the migration.