As of 2.19
You can edit manufacturer part costs on the BOM tab of a released revision with a Custom Permission for permission sets.
Contact your Salesforce Administrator to be granted permission.
Salesforce Administrators - Create a new permission set or edit an existing set to grant permission to use the Edit Cost button
- Go to Setup, select Permission Sets
- While viewing a permission set, navigate to Custom Permissions and select Edit
- Add "Cost Edit" permission. Assign the permission set to appropriate users.