Edit: Hydropower generally builds a huge dam to support a large head of water piled on top of it's self. Pumped storage replaces that huge column of water with hill so you only need a relatively small amount of water to store a lot of energy.

Anyway, I still think your numbers are a little high as Solar and wind are never going to have zero output over a day which reduces peek demands significantly. Existing dams can also shift production to meet different sessional needs and act as backup power production. Still, let's call a ~200 billion investment enough to solve grid storage.

Further, current pumped storage can make money though arbitration. So, private investment subsidized by say 2 billion / per year or 80 billion over 20 years seems to make this a non-issue.