J&R offers the Kidde Ionization Micro Smoke Alarm, model no. 0914, for $6.99. J&R Rewards members receive free shipping. (If you're not already a member, click here to join; it's free.) That's the lowest total price we could find by $3, outside of the price below. It features an ionization sensor and requires one 9-volt battery (included).

A close price: Amazon offers it for $7.44 with free shipping via Prime. Sales tax is added where applicable.