Nobody knows what storage would cost because there is no technology that currently exists to make storage possible. Battery technology helps a lot to stabilize the grid over seconds to minutes or even hours, but the kind of storage that would be needed to be 100% solar and wind just doesn't exist. Pumped hydro is great but there are not many places where it is feasible.

So the reality of the current situation is that solar and wind are cheap now since when they are a relatively small percentage of the power and you can fall back on gas and coal storage isn't really needed. But in the future the costs of solar and wind will skyrocket since you'll need to overbuild the capacity by a huge amount, plus build lots of expensive transmission infrastructure, to work around the fact that storage technology at the appropriate scale doesn't exist yet.