The price of the droid spare battery from verizon.com it is $39.99.
http://www.verizonwireless.com/b2c/store/accessory?action=accessoryDetails&archetypeId=11652&page=1
http://www.verizonwireless.com/b2c/store/accessory?action=accessoryDetails&archetypeId=11652&page=1