Home Depot offers the Makita 18-volt LXT Lithium-Ion Battery 2-Pack, model no. BL1830-2, for $99.99 with free shipping. (Scenterprises via eBay charges a few cents less.) That's the lowest total price we could find by $20. They purport to recharge fully in 30 minutes.
Update: As a reader found, Amazon offers it for $99 with free shipping.