Depends on what you want to do, but overall not much. Standard lines hung on towers run from $1 million to $4 million a mile, depending on terrain and other factors.

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.