How can I on the iOS App Store see what an app that I've already bought costs at the moment?
You could use an app like AppShopper, which shows the current price of an app as well as past price changes. AppShopper also has a website where you can find the same information, for example for GarageBand: http://appshopper.com/music/garageband (you can change the country/currency at the bottom right of the page, past price changes are shown in the “app activity” box).
The only way to know the price from the Store requires that the specific app is not installed on your device, and not being signed in. Details:
If the app you want to know the price about is not installed on your device, then:
- Open the App Store app. The app opens with featured section highlighted.
- Go to the bottom of the list in that section.
- Tap on your Apple id.
- Tap on sign out.
- Force-close the app (the App Store app) i.e. open the multitask pane and swipe the app out of the screen.
- Open the App Store app again.
- Look for your app and you will see its price.
If the app is installed on the device, you could delete it and follow the above process.
If you wouldn't like to uninstall (and possibly install again), then the best option is watching the result from googling itunes appname, where appname is the (approximate) name of the app.