I was doing some just-for-fun research tonight and stumbled across this. Renogy has dropped their lithium-ion battery price considerably, and seems to have introduced a new model since I was last in the market. (As best I recall, their 100-watt-hour units used to be about $900.) No, it's not as cheap as do-it-yourself options, lead acid, or things like prism cells. But still, it's a significant drop. I'm sorely tempted, to be honest.
https://www.amazon.com/Renogy-Lithium-Iron-Phosphate-Battery/dp/B075RST4R2/ref=cm_wl_huc_item?th=1
https://www.amazon.com/Renogy-Lithium-Iron-Phosphate-Battery/dp/B075RST4R2/ref=cm_wl_huc_item?th=1