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—-- JS

jQuery(function() {
    initSubtotlaCalc();
});

// subtotla calculation init
function initSubtotlaCalc() {
    var qtyInput = jQuery('.quantity .qty'),
        singlePrice = document.getElementById('product-single-price'),
        dozenPrice = document.getElementById('product-half-dozen-price'),
        subtotalPrice = document.getElementById('subtotal-price');

    qtyInput.on( "input",function (e) {
        if( qtyInput.val() < 6 )
        {
            var svalue = qtyInput.val()*singlePrice.innerHTML;
            subtotalPrice.innerHTML = svalue.toFixed(2);
        }else{
            var dvalue = qtyInput.val()*dozenPrice.innerHTML;
            subtotalPrice.innerHTML = dvalue.toFixed(2);
        }
    });
}

—-- HTML

<span class="price"><span id="product-single-price">2</span></span>
<span class="price"><span id="product-half-dozen-price">1</span></span>
<span class="price"><span id="subtotal-price">2</span></span>
<input type="number" class="input-text qty text" value="1" title="Qty"/>