This app was mentioned in 4 comments, with an average of 1.50 upvotes
Literally every app.
Go get Kitco Gold Live
Install.
Go to your Settings -> Application Manager
Scroll down to Gold Live.
Go into it and go to Data Usage
Scroll down until you see Restrict Background Data
Check it.
There ya go.
It won't pull any data unless you're on WiFi.
You can set this for any app.
Kcast Gold Live! is an app I use all the time. Real-time updates of precious metal prices, oil prices, and other metals like copper and aluminum. I am an investor in some of these metals, so I might get more use out of it than someone who isn't an investor. Regardless, it's a very interesting app if you're an information junkie or a subscriber to /r/DataIsBeautiful (the app has lots of cool graphs and statistics).