I have a document with 2 different options to choose from. Upfront payment, or monthly repayment.
Make sure the choice values of the radio buttons are set to Choice1 and Choice2. You'll find the Radio button Choice value on the Options tab of the radio button properties dialog, the
Add this code to the Calculation script for the field that will display the value.
event.value = this.getField("Total Cost").value;
if(this.getField("RadioButton").value == "Choice2")
event.value = event.value/36;