I hesitate to mention this because I don’t want the ebay seller to sell out before I decide if I want one, but…

Craft Computing has a recent video about a used Supermicro “Microcloud” server that holds 8 Intel socket R nodes in 3U and costs $400 (apparently including CPUs but not RAM). Seems like an excellent way to get cheap redundancy, albeit at the cost of probably not great power consumption because it’s so obsolete.