Any app made by We Want To Know is worth owning, from Numbers to Dragonbox 12+. All of my kids, ages 4-14, love all of the games (yes, my 4-year-old understand algebra concepts!), and I love the brilliant ways math concepts are taught. Big Numbers is no exception. You gather resources and build lands, adding and subtracting everything as you go. The numbers get bigger and the problems more difficult, and it is all so natural that the kids hardly realize the difficulty increases--the next steps come naturally. Another nice feature is that the kids have to write the answers, at first guided by tracing a number, then on their own.
A fantastic app, worth every penny!