Tax Label not updating in Price Set using _buildAmount Hook
I finally found my own answer. This is a known bug for text fields in price sets:

https://issues.civicrm.org/jira/browse/CRM-14823

Workaround as per the above link: 

https://github.com/fuzionnz/civicrm-core/commit/3174c2e183bc8cfab9082353966a994a9a364bbb