With orders of $25 or more, Amazon offers the Duracell AAA Rechargeable Stay Charged Battery 4-Pack for $7.99. Clip the $3 off coupon on the product page to cut it to $4.99. Plus, Prime members receive free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it starts around $5.) That's the lowest total price we could find by $6. These pre-charged 800mAh AAA batteries purportedly retain power for up to 365 days while not in use.

Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.