Even if we rebuild the whole continental US power grid (that's a lot more than $50 billion) to cope with the demands of using storage, it is unclear to me if we can build enough hydro to replace peaker plants. Apparently all other storage technologies other than hydro are cost-ineffective and/or do not have enough scale to solve the problem. Do you know if there is enough geography to go around for all the hydro we need or if there really are any cost-efficient alternatives?
The economics for ~100% up-time only work out when utility's are regulated to act that way. That means spare capacity and backups for your backups. As such I feel a few Peaker plants are actually a good idea. They should not be turned on every day and should be able to pick up the stack when unexpected things happen.
In terms of hydro, we can build them to store ~12 hours worth of US electricity demand. Past that the geography still works just fine, but excess production becomes cheaper than excess storage.
PS: Don't forget we already use pumped hydro. It's just a maximization problem where cost of excess capacity vs. extra storage.
Though, we already have an electric grid and you can do a lot by simply shifting where each area's supply is coming from. AKA town A get's power from town B's power plant town B, get's power from town C's power plant etc.
Also, a lot of this infrastructure is a ~20+ year problem. So, a 50 billion dollar bill spread over 20 years spread over the entire US is ~8.50$ per year per person.