Amazon offers the Energizer Lithium Coin Watch Battery 2-Pack for $2. Add it to your cart via Subscribe & Save to drop that to $1.90 and bag free shipping. (Walmart charges about the same.) That's the lowest price we could find by $5. They're used in heart rate monitors, keyless entry, glucose monitors, and toys.

Be sure to cancel Subscribe & Save after your order ships to avoid being billed for subsequent shipments.

Update: It's now out of stock at Amazon; however, Walmart still offers it for the same price with in-store pickup.