Battery modules (not home scale, but utility scale) are around $50/kWh in China. If we assume a 20 year lifespan and 50% charge/discharge once a day, that adds (ignoring interest) $0.013/kWh to the energy cycled through the batteries (plus a small add from efficiency being not quite 100%).

This is quite cheap compared to (say) the fully loaded cost of energy from a nuclear power plant.

Smaller units will be more expensive per kWh, but not so enormously so as to render them impractical. And they will get cheaper quickly like all electronics do.