Amazon offers Prime members the SpareOne Prepaid Emergency GoPhone for AT&T for $38 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $6.33.) That's the lowest total price we could find by $10, outside of the price below. It runs on two AA batteries and provides up to 11 hours of talk time. Features include a glow-in-the-dark keypad, flashlight, panic siren, SOS signal, and a locate and alert service.
A close price: Walmart charges $39.99 via in-store pickup.