In this specific context, I don't think appraising the total money Sergey Brin has given matters. I'm sure he has enough money to fund any/all decent research proposals. The decent proposals must come first though, one can't simply say he should be throwing X total number of dollars at a problem. Throwing more money at things does not always makes things better and sometimes can make things worse.
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I totally agree with you on the importance of targeting, but I wouldn't go as far as to say that the "amount of money [...] doesn't matter".
A lot of money going to really good things is still better than a little money going to really good things.
In this specific context, I don't think appraising the total money Sergey Brin has given matters. I'm sure he has enough money to fund any/all decent research proposals. The decent proposals must come first though, one can't simply say he should be throwing X total number of dollars at a problem. Throwing more money at things does not always makes things better and sometimes can make things worse.