Why are solar batteries so expensive?

By “solar battery” I assume you mean batteries for homeowners which store electricity from rooftop solar panels during the day for use later after the sun sets. Such batteries cost much more than the $100/kWh we often hear quoted for EVs. There are three main reasons for this:

1. Cycle life. An EV with 300 mile range only needs a battery to last 800 full charge/discharge cycles to deliver a 240,000 mile operating life. To provide 15 years of daily service a home battery system must survive 5000 cycles. That costs extra.
2. The car battery price only includes the battery pack itself, or often just the cells. A home battery price includes an inverter, often a cooling system, consumer interface software and hardware, etc.
3. Wholesale vs. retail. Carmakers buy thousands (soon millions) of battery systems per year. You are buying one, or perhaps a few.