Not sure why the prices are so much higher when compared to EV batteries, but now that Na-ion batteries are in production we should see a significant reduction in prices for house backup battery systems over the next few years - 1/2 to 1/3 the cost of Li-ion. And while Na-ion has less energy density it's probably better suited for home power backup (less likely to catch fire, works better in low temps, for example) where you don't care about the extra weight.