When playing with friends, Battlescribe is usually fine but for a tournament you need a legal copy. These cheapest method (in a way) is the Gamers Edition that GW put out on iBooks. I think it's around $17. Might be more. But you would need an Apple iPad or computer to read it. Might work on an iPhone. Not sure about that.
The Gamers Edition doesn't have the fluff (background information and stories), just the rules.