have you tried using "bettery battery stats"? that's supposed to help.
My solution was just to get an extended battery this one. It's been really great. The first one they sent me didn't work, but the second one did. It's double capacity of the stock battery. I usually get 3.5 hours screen-on time. Even if the battery busts and i need to replace it, it would still be worth it if it lasts every 6 months. The battery itself is just 10$. The link includes the TPU case.
I bought mine for 60, 9 months ago. That was a really good price. I had to use all my ebay shopping skills to do it.
The smooth coating on the glass was worn off, so I had to buy a glass protector for $10. Without it, it collects fingerprints and oil fast. I don't think people take that into account when buying a used phone. But I'm still happy with my purchase. I wasn't really happy with the battery life I was getting and I eventually just got an extended battery (this one) and I love it. So I put an extra $26 into it before I was genuinely happy with it.
$50 is a good price.