Calculate A Price Based On Customer Income

We are trying to create a fee entry for a customer, and we need to base that fee on a % of the customer income.   For example… Product A costs 1% of the Monthly Customer Income, so if the customer income is $1000, we would charge $10.00 in the fee table.  

The process right now: 
1) Present the employee a list of fees (Fixed, Adjustable, % of Income) via Pop-up
2) Employee selects the fee to charge
3) Create New Record in Customer Fee Table
4) Copy in all the Fee Schedule Values to the Customer Fee Table (no reference, in case the fee changes later)
4) Save The Customer Fee Table
5) Close Pop-Up

We are trying to use: 
(({{Cost_Amount__c}}/100) * {{$Model.CustomerFees.data.0.Customer_Income__c}})

However this will only display the formula out…  1/100 * 1000  

Is there a way of doing this when we are inserting the records?   All our other scenarios have been able to be addressed with the Actions, but this is the last one.

Thanks!