I'm embarrassed by the 'basicness' of this question, but after wasted hours, here goes.

In an effort to do something with the jQuery Calculation plugin, I am playing with the basic example of the order form on the plugin site. I want to have the grand total as a form field,rather than text, so I can use the value.

The function that calculates and shows the grand total is:

    function ($this){
                // sum the total of the $("[id^=total_item]") selector
                var sum = $this.sum();

                $("#grandTotal").text(
                    // round the results to 2 digits
                    sum.toFixed(2)
                );
            }

the total updates on keyup in:

<span id="grandTotal"></span>

But this does not work with:

<input type="text" id="grandTotal" value=""/>

Can anyone point me to what I need to add/change to make that work? To call my javascript basic would be a compliment, so please talk to me like I know nothing!