True that both battery systems and, more surprisingly, automatic transfer switches are pretty expensive. But I think a lot of that still has to do with lack of demand and competition. There are literally only 2 or 3 providers that I could find that make automatic transfer switches. and it was a minimum of $800. I ended up going with a $40 AliExpress option that is supposedly able to handle 125 amps which is enough for my needs but isn't UL listed, so I think would only work for diy. I also built my own battery and inverter system for about $2500 in parts that is about the size and power of a Tesla Power wall which they sell for more than $10k last I checked. but again, inverter is not UL listed.