function summOrder()
        {
            summ = 0;
            var trs = document.getElementById('order-table').getElementById('order-table-tbody').getElementsByTagName('tr');
 
            for(var i in trs)
            { 
                var price = isInt(trs[i].getElementById('item-price').innerHTML);
                var val = isInt(trs[i].getElementById('item-input').getElementsByTagName('input')[0].value)
                
                if((val>0) && (price>0))
                {
                    summ += val * price;
                    alert(summ);                    
                };
 
            };
            
            alert(summ);
            return summ;
        }

document.getElementById('order-table').getElementById('order-table-tbody')
...
trs[i].getElementById('item-price')
...
trs[i].getElementById('item-input')

document.getElementById('нужный_ИД')