Referring to @GiantTree 's comment, and adding on some of my own experience:
Google Play Store does its utmost to help you save data while updating apps.
App updates are usually about 20-30% of full size.
If the app does not contain .obb (expansion) files, it will use file patching techniques to update the .apk in your data partition. For the first update of a system app, the updated app is downloaded in its entirety once to /data/app, as the /system partition is read-only on unrooted devices and file patching cannot be done there. Also, once this first update has finished, file patching techniques are applied to reduce the amount of data needed to download.
This is not applicable as a whole to applications with .obb files, as these are stored on external storage and have a different, independent patching mechanism.
Updating an app through the Play Store saves you data already, in fact Google considers it a violation of its Terms of Service if developers who distribute apps through the Play Store employ means to update it other than Google Play's own mechanism.
For the moderators, I found a source (Although this is for APK expansion files):
Updating Your Applications-For Developers
And this, from Google Play's ToS:
An app downloaded from Google Play may not modify, replace or update its own APK binary code using any method other than Google Play's update mechanism.
From: Google Play Developer Content Policy
And this picture: