sealed lead acid ($0.52/WH)

nickel metal hydride AA ($1.08/WH)
$3.7 3.4WH 2300mAh 1.5Vout
http://www.atbatt.com/product/8292.asp

lithium-ion ($2.2/WH)
$100 44WH 5/11Vout
AC-Chargable
360 grams
http://www.bixnet.com/mumiexbapaki1.html

AA batteries are a win over li-ion. That's a surprise.