Batteries for my old samsung are running about $5.99 on ebay plus $5.99 shipping. Can't beat that price, and many of these merchants have 1000s of + feedback. ^
Be sure to check the capacity so that you are comparing apples to apples (or in my case 1400mh to 1400mh) - you may find that some batteries which appear cheaper are in fact either reconditioned or have less storage capacity.
Or you could just go to the sprint store (or whatever carrier) and pay $59.99 for a replacement. Personally I prefer ebay but they're your apples.