JQuery: How to get selected radio button value?

$('input[name=myradiobutton]:radio:checked') will get you the selected radio button
$('input[name=myradiobutton]:radio:not(:checked)') will get you the unselected radio buttons

Using this you can do this

$('input[name=myradiobutton]:radio:not(:checked)').val("0");

Update: After reading your Update I think I understand
You will want to do something like this

var myRadioValue;

function radioValue(jqRadioButton){
  if (jqRadioButton.length) {
    myRadioValue = jqRadioButton.val();
  }
  else {
    myRadioValue = 0;
  }
}

$(document).ready(function () {
  $('input[name=myradiobutton]:radio').click(function () {   //Hook the click event for selected elements
    radioValue($('input[name=myradiobutton]:radio:checked'));
  });

  radioValue($('input[name=myradiobutton]:radio:checked')); //check for value on page load
});