Amazon offers its Amazon Prime members the Duracell Pre-Charged Rechargeable AA Batteries 4-Pack, model no. AA-RCHG-STAYx4, for $8.99. Clip the $1.50 off coupon on the product page to drop it to $7.49. With free shipping, that's $1.87 per battery and the lowest total price we could find by $6. (Non-members can spend $25 or more to qualify for free shipping; otherwise, shipping costs $5.14.)

Amazon Prime membership is $79 per year; however, Prime is free for college students for six months, and free for parents and caregivers for three months. (Note that currently there's a waitlist for Amazon Mom.)