I went with the $5-$10 Ebay batteries thinking I was getting away with a super deal replacing Iphone batteries before.. but it was quickly apparent that they weren't much better than the used up battery I was replacing.
So I shopped around, and to my surprise the BEST price for a genuine Iphone battery (non knockoff) was from best buy.. So I ordered it online.. something like $20-$30,.. and it arrived quickly, installation is about 5 minutes, and The phone holds a charge for a week now instead of a day like before. With heavy use it's still a few days before a charge is absolutely needed.
Great price, and the real deal.