I just bought a genuine battery off ebay for £6.49. I have bought batteries for my 6600 and n80 from the same seller in the past and although the batteries are genuine they tend to be from returned faulty phones rather than brand new. I have found this to be a cost effective way of buying genuine nokia batteries and have reccomended it to friends too. On occassions where batteries have had an issue (ie not holding charge) the seller has given a prompt replacement or refund and the second battery has had no issue. So, you don't have to pay a vast sum to ensure a genuine battery, just be careful who you buy from!