those 18650 lipo batteries many packs are made from are $3/pc online. Used to be 1/2 that.... 10X 3.7V 14500 18650 26650 Li ion Rechargeable Battery + Charger Set | eBay
try and find 'em with the tabs so you can solder them. Many Ebay listings must include a charger or a case or something so they can get around shipping them bare.
