I am on a 100MB/month plan which I primarily use to check email but I had been getting large downloads happening in the background.  Steps I did to limit the amount of data include the following:

1. Use the My Data Manager app.  Though there is built-in data usage tracking within Lollipop, the app provides notifications and more detailed statistics.
2. Configure Play Store to not auto-update apps.  This option is set within the Play Store app.   Menu bar icon -> Settings -> Auto-update apps.  You can set it to update over wifi only as well but I turned it completely off.
3. Disable sync settings for Google Accounts.  This is under Settings (main) -> Accounts -> Google -> Your account name.  I turned off sync for everything except Gmail.  Android tends to install new google features which you will need to disable the sync for.
4. Optional.  Restrict Background Data.  I’m still unsure if gmail will be checked in the background if this is turned off.  I have left it enabled but to disable it, go into Settings (main) -> Data Usage and it is hidden under the extended menu with the 3 dots in the top right corner.  You will have a warning in your main notifications  with it disabled.
5. IMPORTANT.  Turn off Google Play background data.  This option was probably causing my large downloads.  Go into Settings (main) -> Data Usage and find Google Play Store under App Usage.  It needs to download some data first in order to be displayed in this list.  Click on Google Play Store and you will have the option to Restrict app background data.  Repeat this step for any other app that is downloading large amounts of data in the background.