$3.36 INC DELIVERY FOR A HUNDRED AG3 BATTERIES SO ROUGHLY £1.70 = 1.7p EACH
AG3 batteries are the button cells that are used in many small LED torches and most laser pointers. WIth prices in the UK most people find it cheaper buying a new torch or pointer than replacing the batteries.
However, with 100 for £1.70 delivered (and discounts on bulk) there is no need to bin the equipment when the batteries die. It might sound daft - but check these are the correct type before buying them!
Lastly - if you can get ones that run on rechargeable AAAs they will last far longer and overall cost you far less - in addition to vastly reducing the waste.