[quote=TheDookieNut]
Apps are more inclined to using a LOT more data. Push notifications, running in the back ground. I get 250mb of data a month and have never exceeded. Try turning your data off when you don't use it. That's always an option
[/quote]

Depends on how push notifications are coded. On Android, if they're sent through Google Cloud Messaging, they don't incur any real data cost until actual data is being sent by the app server. I believe iOS has a similar method for push notifications as well. Obviously, if you're doing home-grown push notifications from the server without any OS-level assistance, yes, you're going to have massive data spikes. 

Running in the background, however, doesn't cost data. Only memory, which isn't something phone users are charged for on a monthly basis.