Monoprice, a leader in value-priced accessories, offers its own version of the iPhone 4/4S extended battery for a mere $23.63. We can't say it gets huge points for style (not sure a lot of folks want to rock the Monoprice brand on their iPhones, but hey, to each his or her own). As its name implies, it has a 1800mAh battery.
It's unclear from the product page whether this case is compatible with both the 4 and 4S, but customer testimony seems to indicate it does (we have not tried this case).
As one customer says: "Although the product is described for the ATT version is does fit and work well with the Verizon 4S. The only button that is awkwardly placed for the case is the ringer mute button which is partially hid by the case but still accessible and functional." (It also appears to work with the Sprint and AT&T 4S with the same caveat).
See pricing for Monoprice 1800mAh Backup battery case from monoprice.com.