Radio button that change the value of the text field

Hy everyone,
 
Iv'e look out for about three hour to this question and I did not find it.
 
First of all I use Adobe lyvecycle designer ES vers. 9.0.0 that is include with acrobat X to make a form
 
Here a script that I manage to create with the help of this forum. (It should tell that if you click yes on the radio button it will write 200$ in the text field) I will repeat the same script for the 'no' radio button.
  
topmostSubform.Page1.Transport.#field[0]::mouseUp - (JavaScript, client)
//The only way i'm able to create a script was by using the script editor. I had to choose a option so I took the 'mouseup' option. But is there another way (better way) for doing it
 
var choice = this.getField("Transport").value;
//transport is one of my radio button
if( choice == "0" ){

document.getElementById("Formalites").value = "200";
//formalites is my text field
}
else {

document.getElementById("Formalites").value = "0";
// the 'else' section is quite useless cause they are 2 radio button and I will put the '0' value at the other one in replacement of the '200'.
}
  
I never use javascript before but i'm use to PHP laguage. Thank you for your'e future responce.