Judging an act by its consequences seems like a frightfully hard-nosed, rational thing to do; the problem with trying to put it into practice is that we tend to end up judging only by the superficially obvious consequences and ignoring the more important long-term effects.
It's easy to compare the nominal cost of this event with the idea that it only helped one person, but the latter isn't actually true; it obviously brightened the lives of thousands of people, and not just with shallow entertainment but with the opportunity to lift their gazes from the mud and look at the sky, to reflect for a moment on their place in the world. I have no idea how to calculate the benefit from that, but it wouldn't surprise me if it greatly outweighed the cost.
As you say, truckloads of aid money has been pouring into Africa for decades, and not only has it not produced the promised results, it has been argued by people familiar with what actually happens there that it does more harm than good: http://www.spiegel.de/international/spiegel/spiegel-intervie...
By contrast, smartphones are connecting Africa to modern civilization, and that's something that might actually make a positive difference. There is after all no intrinsic reason Africans can't provide food and water and medicine for themselves just as people do in Europe and America and large parts of what used to be the Third World. What's missing is knowhow and organization. A truckload of money dumped into projects that feel good to rich Westerners won't provide those things. Putting the knowledge of humanity at the fingertips of individual Africans just might.
And yet those phones weren't paid for by charity, because the people running charities are mortal and fallible; they just can't see that far ahead. The phones were paid for primarily by rich Westerners buying them as toys.
The world is more complex than any of us can know. Enjoy your toys with a clear conscience.