I'm not sure how many old xeons I could buy with 1 grand (including associated memory sticks, HDDs and racks) but I'm pretty confident it's going to assemble more than 10 servers. If we get mobos with extra sockets that pushes the core count up even more.
But then didn't I even give more optimistic numbers than what you're suggesting? Sure, old chips might draw a bit less, but not by a lot. By them being significantly cheaper than the latest and greatest Ryzen, you can get a ton more of them at the same cost of a specific ASIC, so even if they draw a tiny bit less, in total the consumption will go even higher than the 600W/$1k I suggested.