I'm not saying this is the correct amount, but UN [1] says 30 billion/year would be enough to do it. However, I believe that they are taking into account only the financial needs and not the political effort necessary to do it, mainly to prevent people from diverting that money from going into the right place.
I wasn't going to get into this subject to prevent a long debate, but I'm always amazed on how these things go. We have all the money in the world to fix it for good, but for some unknown reason we just can't do it.
There was a time that I thought that if someone as powerful and "rich" as Musk ran for president for some big and important country (like the US) they could fix everything.
But for some reason that is unknown to me this will never happen. And when something close to it (in the power and money sense), like Trump running for president, does happen we know that we are not going to get this "magic fix".
It seems that at the moment that the possible fixer gets to a position where he can fix things, he no longer wants do it.
Another stupid idea, or parallel, is Pablo Escobar. At some point in his life the guy spent 2k+/day just for money rubbers. At first he wanted to be good and do good for Colombian people, but when he got to a position where he could do it, he no longer wanted to do that.
I guess we will never fix anything and the world will be as screwed as it is today. Or worse.
[1] http://borgenproject.org/the-cost-to-end-world-hunger/