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.
For $800 you can get a complete 2kWh system, including cells, inverter and panels shipped to your home in Europe, so if you're paying more than that for half the storage either your government or contractor is fleecing you.
I can confirm that prismatic 1kWh LFP cells cost ~$60 in single digit numbers.
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.