$10 4800mAh Battery at SuperUFO.com
Considering the included battery is only 1800 mAh, this is a nice deal... Not sure what shipping is, though... maybe cheaper elsewhere, but $10 sounds good to me!
http://www.superufo.com/pp/product.asp?id={9324FC56-932C-4C40-9442-F694F5EBB01D}