The cheapest barebones I have seen has been for a Gigabyte E252-P30 (CTO option) with the lowest spec Ampere Altra Q32-17 Processor and 8GB RAM with no storage at £4k in the UK and around $4.6k in the US - I've also kept an eye out for the old ThunderX2 motherboards as ThunderX2 procs are almost always available on Ebay but I've never seen a mobo for sale - I suspect the manufacturers are making no attempt whatsoever to get any of these into the general publics hands. If you set up your own newest tech review site I suspect you may have a better chance of getting one
Buying at the lower-end is pretty tough with these unless you are a fairly large customer. The reason is that the platforms are lower volume and thus cost more. The chips themselves are actually not "that" expensive compared to x86 list prices.
I think they need to have the 80-128 core chips priced at no more than $2K ish to compete with x86 at cloud customers. So the lower volume servers and motherboards will end up costing more than the 32C parts.
As noted in yesterday's GB motherboard, another challenge is that getting a heatsink outside of an OEM system is very hard.
I know that is not the best answer, but just something to keep in mind. If you are buying a few racks, then these are now easy to get. If you are trying to buy a single server the prices will be very high because the supply chain is not set up for it.
Actually, this is a great point that we do not cover enough.