The historical trend line https://ourworldindata.org/battery-price-decline, that those involved in this business seem optimistic that this will continue into the future, and that I am not aware of any technical barriers preventing that (even with no technological breakthroughs it seems likely that we would continue to see declining costs for some time even just due to economies of scale).
Expensive for now. I'd expect that to be a cheap battery bank in <10 years.