Case Structure and a Text Box

Help!

I am working with a form that retrieves a value using a case structure;

Case(
fuel_consumption="low" and rate_based_on ="One Month" ;monthly_charge_out_rate_1_low;  
fuel_consumption="med" and rate_based_on ="One Month";monthly_charge_out_rate_1_med;
fuel_consumption="high" and rate_based_on ="One Month";monthly_charge_out_rate_1_high)

Now according to which ever radio box is selected, Low, Medium, High, fuel_consumption value is changed. The problem is the only value that shows is the high value. Should I be using something like GetField to retrieve the value at the end and why isn't my text box receiving the evaluated case calculation?

Thank you!

Kevin