Formula Help, sending me mad

Formula Help, sending me mad

I want to write a simple formula i have two fields that i need to add together in CRM cases. 

I am trying to create a custom formula field that adds these fields up 

I enter 

${Cases.Administration Fee}+${Cases.Final Duties Fee}

I recieve the error   Formula Custom Fields not allowed in expression 


Can anyone show me the correct way to write this formula 


Thanks 



Chris