Yangdock E-commerce via Amazon offers the Masione 3,500mah Roomba Replacement Vacuum Battery for $19.99. Coupon code "AK9UB4RP" cuts it to $14.99. Plus, Prime members bag free shipping. That's $1 less than last month's mention and the lowest price we could find by $2. It's compatible with a range of Roomba models (listed on the product page).