Does this sound like a good book idea? TEN POINTS BEST ANSWER!!!?

It sounds great. Although I agree that it's kind of cliched, there's a reason for that: people LOVE reading about ethical situations like this, where people have to choose between what's right and survival. However, if I were you, I would throw in a huge twist at the end, such as Arianna and her friends (or whoever survives) make it out of the desert, only to find out it's a government conspiracy to weed out those too weak to survive in the new world they're planning to build, or something. And maybe include some romance or something? But it sounds good to me!