I forked out £4.99 (at the time) for the "Train Times" official National Rail app!
To this day it's been worth far more to me than that, I use it multiple times when on shift at the Gateline to supply information to passengers.
It is often the most reliable source of live departures and journey planning around, I am aware that you can get this same information for free by browsing sites, but that is not what I want, the app displays it much nicer and is very quick to use.
You can't buy tickets with it, which is a shame, but not what I needed the App for. So for a passenger it is debatable whether it's worth spending money on such an App!