I'm not sure I understand the question. It's always cheaper.
Storage is more expensive than generation. Always.
We see some storage on the grid because storage is fast. Batteries can respond to load changes a lot faster than anything else. Demand goes up, the batteries are drawn down while cheaper generators are spun up. A few places use pumped hydro for the same purpose, but that's very situational.
For example if nuclear costs 1bn for a GW and solar+wind costs 0.6bn but you need to also install 0.5bn of storage to make the grid reliable to the safety margin you deem acceptable then it is no longer a cheaper system.
Amount of storage needed scales up rapidly as you get towards an grid with entirely intermittent generation.