You can get a non-Threadripper Ryzen CPU+mobo with ECC support for far less than $400. Obviously it lacks the PCIe lanes of the TR setup. These are both ridiculously cheaper than you could get similar capability for before Ryzen launched. If you really want 48 PCIe lanes for $400 you can get a used Xeon X99 setup for that much.

$750 for "workststation" class is really inexpensive.

$400 is really cheap even by consumer standards.

Really? I don't need high clock speeds, I'm looking for a reasonable (48) number of PCI-E lanes and ECC support.

That's called workstation class. Most consumers need barely 24.