Just because Bono makes more money than us doesn't mean his time is more valuable than ours. If anything, our time is more valuable than his since we have to spend more of it to make as much as he does in a shorter period of time. (i.e. *simple exaggerated example* he makes $1 mill in a year. it would take me 20 years to make $1 mill. I see that as MY time being more valuable! Yes he makes more money in a shorter amount of time, but it takes me LONGER to do the same).
Anyway, that's crazy- everyone's time is valuable, not one's more than the others, b/c time is something you can't get back, and you can fill it up with priceless things.
To the original question- IMO, it's irrelevant how much he gives, if anything. Money is relative (much like the time issue) so $100 to you may be like $1000 to him. AND, on top of that all, even if he were to give his entire fortune to DATA, it wouldn't make a difference (nor is it his responsibility to do that!). Africa pays back $1mill a WEEK. Bono'd only be throwing pennies on the fire.