I wouldn't suggest deleting apps, at least not right away, as there is the potential to break the phone and storage space isn't an issue with 10GB available. Just disable the ones you don't want/need using the "Turn Off" button which is available in the App Manager for a lot of the preinstalled apps. If that button is there the app is definitely safe to turn off. Other apps can be "frozen" using Titanium Backup if you're rooted but be careful with system apps. If bad things happen when freezing them they can be turned back on.
Other things you can do-
Keep the screen brightness down to the minimum possible setting.
Turn off un-needed radios- Bluetooth, WiFi, GPS, NFC as they will use some power just by being on. Turn each one on only as required. If you're inside with a good strong wifi network use wifi calling and it will turn the cell radio off.
If you are going into a location where you know there won't be any signal, such as a basement, subway etc. put the phone in airplane mode so it doesn't use the battery searching for a signal.
Disable automatic syncing for accounts and individual apps unless you really want them to do so. Syncing uses battery even when the phone is in standby. Many apps have push notifications turned on by default, go into their settings and turn it off. Same for email, use manual mail-check settings if you can instead of push or automatic interval checking.
You can go into Settings>Battery to see a list of what functions and apps are using the most battery. Typically Screen, Cell Standby, Android OS etc will be at the top with individual apps further down the list. If something is using an unusually high percentage of battery you should see it here. Also in the App Manager you can look under the Running tab to see what apps and processes are running in the background along with how much RAM each is using (although CPU % would be more important to determine battery use). The Active Apps widget can be put on a homescreen so you can see what apps are running in the foreground. Don't try to kill running apps and processes in an attempt to save battery, either manually or with "battery saver" or "task killer" apps, this doesn't help and actually makes things worse. Just let Android do its thing.
I do the above with all my phones and it all helps with battery life. Having taken these steps on the Avant I find it's doing fairly well, especially on standby where the battery percentage will remain the same for a long time if I'm not touching the phone.
As for the cost of aftermarket batteries, both regular and extended, this will vary considerably due to quality, where they come from etc. Unfortunately since none of these are OEM you're taking your chances no matter what and the real cheapos may not last very long or might not have their advertised capacity.
