Gotta ask ... "if the batteries do last their claimed cycle life (in one test, only 5% of brands did.) " ... So which brands were in the 5%?
"While many battery packs have experienced faults and/or failed prematurely, the Sony battery pack from Phase 1 has proven highly reliable, alongside the Pylontech and GNB Lithium battery packs from Phase 2. The Sony battery pack (Phase 1) has retained over 80% of its initial capacity after nearly 3,700 cycles. The Pylontech battery pack (Phase 2) has also retained over 75% of its initial capacity after nearly 2,800 cycles."
Cost is irrelevant to sale price. market value is king. profit drawn from building, marketing, future sale reduction pricing, etc. I would be EXTREMELY surprised if sales were anywhere close to material and labor and manufacturing costs... It's like the cannabis market. It costs MAYBE $50 to...
diysolarforum.com
But prices today may be $0.10 on the dollar compared to then, so you can better afford the risk.
I think server rack batteries may pencil out to $0.05/kWh over 6000 claimed cycles (16 years). DIY batteries similar even if half the cycles assumed.
Two years ago, I looked up LG RESU for use with Sunny Island. 5x the price of SunXtender AGM, and 5x the cycle life.
For my grid-backup system I spent less, still expect to use only a fraction of cycles over 10 year life.
Meanwhile, recalls on RESU due to fires (also on Chevy "Blaze", I mean "Bolt" batteries.)
Today, 280 Ah LiFePO4 DIY would be half the price of SunXtender for same usable capacity, and 100A LiFePO4 server rack batteries same price per usable Ah.