If I run this code:

    var web = new HtmlWeb();
    var doc = web.Load("http://www.overclockers.co.uk/showproduct.php?prodid=GX-033-HS");
    var priceContent = doc.DocumentNode.SelectSingleNode("//*[@id=\"prodprice\"]");
    Console.WriteLine("price=" + priceContent.InnerHtml);

It outputs:

price=529.99

So it seems to be working. You can also use //span[@id=\"prodprice\"]" which is better as it avoids all non SPAN tags.