[jQuery] how disable text box based on "select" setting

Hi

I am new to jQuery - appreciate some help.

I found some code that hides a textbox when a dropdown is set to a
specific value. That works fine.

$(document).ready(function(){
if($('select[name=costType] :selected').val() == 'fr'){
$('input[name=costValue]').disabled();
}
});

What I would like to do is not hide the textbox but disable it. I
would also like text box's contents cleared when it is being
disabled. If the user goes back and selects a different value in the
dropdown, the textbox to be enabled, with no value in it.

The dropdown name is costType and it has 3 possible values. Only one
of them is to trigger disabling the text box.

Thank you for your help
Steve