As hunny said, it's very difficult to go through 15GB per month.
However, if we assume that you do and that you also get some kind of notification (email, SMS), you can use an app called Tasker which has the functionality to switch things on and off. The only caveat is that you will need to do some tweaking yourself.
If the notification is also on email, you can use ifttt.com (to read the incoming email), which can then send a message to Tasker to switch off your WiFi and also notify you. The possibilities are then endless if you get that far.