Another jquery calculation question.

I've this, which is sample code from the plugin site that I am playing with to get this working:

        function recalc(){
    $("[id^=total_item]").calc(
        "qty * price",
        {
        qty: $("input[name^=qty_item_]"), 
        price: $("input[name^=price_item_]"),
        },

        function (s){ return s.toFixed(2);},

        function ($this){ var sum = $this.sum();

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

Changes in the price fields cause the totals to update:

        $("input[name^=price_item_]").bind("keyup", recalc);
        recalc();

Problem is I won't know the value of the price fields, they will be available to me only as a substring of values entered by the user, call it 'itemcode'. There will be a variable number of items, based on a php query.

I've come up with this to change the price based on the itemcode:

    $("input[name^='itemcode_item_1']").keyup(function () {
    var codeprice = this.value.substring(2,6);
    $("input[name^='price_item_1']").val(codeprice);
});

$("input[name^='itemcode_item_2']").keyup(function () {
    var codeprice = this.value.substring(2,6);
    $("input[name^='price_item_2']").val(codeprice);
});

However while it does that, it also stops the item_total from updating. Also, I feel there must be a way to not need to write a numbered function for each item on the list. However when I just use

    $("input[name^='itemcode_item_']")

updating any itemcode field updates all price fields, which is not good.

Can anyone point me in the right direction? I know I am a bit clueless here, but javascript of any kind is not my thing.