Cheapest option is buying HP Z600 workstation for 150 Euro and fitting two Xeon X5670. Make sure you get the later motherboard supporting these and buy one taht already comes with two CPUs.
Then fit a SSD and a Geforce GTX860. Buy memory.
Total cost is around 400 Euro for 24 virtual cores, 24GB RAM, GTX and SSD.
Can't beat this price/performance ratio.
Regards,
Vitor
this have had me poisoned ever since... in fear of being left out of stock, i grab one Z800 from US seller raw cost $255 (excl shipping, vary with countries) inside are 2pcs Xeon X5675 3.07GHz on HP (custom?) MoBo, 24GB DDR3 ECC RAM, NVidia Quadro 4000 Graphics, 1TB HDD, DVD RW, 850-1000W? PSU, and a thick boat anchor grade unpenetrable casing with nice HP emblem on the side and some wind deflectors / RAM fans / HDD & DVD bays inside. you can get cheaper $200 from the seller if you pick the lowest spec Xeon X5570 2.93GHz 12GB RAM. you can find him in ebay US easy, 2 unit left from him...
unfortunately on mine, the Quadro 4000 is not working properly (broken), so i have to find another card as replacement, a $70 Radeon RX470 is choosen due to its performance/price report in the net.
its new unit, but not a new model (2016?) (corrected: ex-mining unit). comparing the spec,i believe this buy is still worth it even if the old sluggish Quadro is still working. another $25 cheap-arse 120GB SSD upgrade, the bundled 1TB HDD becomes 2ndary. so total $350. info in the net indicating people OC this Xeon up to 4.5GHz stable (with water cooled), but the stock HP MOBO is dumb ass wont let any clock tuning. the OC capable dual Xeon socket Mobo is still out of reach, price wise (or not really worth it for this cheap-arse alternative) so i have to live by the Non OC stock speed but with Turbo Boost enabled.
i'm surprised 14 pages and at least 2 tweakers here, not much report on performance. only the nvme report afaics. for free graphics (game oriented person) benchmark can be downloaded at
https://benchmark.unigine.com/heaven?lang=en so let me show my benchmark first

, i dont expect this one thirdth of a thousand used PC will be any wonder in performance, maybe latest HW can easily achieve 3X of this? but i'll be happy if anything can render at more than 30fps the oldie standard, i wont push myself to be dependent on faster screen refresh rate spec

otoh disk write report is abysimal at 10-100MB/s rate, not sure what they are and not really care. well i mean what can i expect from a $25 SSD connected to old SATA on a 10 yrs old MOBO? i wont go into hassle of nvme/pcie hack, this old system may not support it in anyway, and GBytes of data transfer daily is not what this machine is intended for, so that is not really critical imho.
i dont really have an urgent need for this, i just dont want to miss the 12 core xeon party before its gone. even with shipping cost that i paid, i think i cant get the price, not even close if i try to gather parts from current technology (intel only mind you) that can match the core and ram spec. the purpose will be mainly for its core number to run some homebrew simulation if i need to, and to run investor/fusion those heavy stuffs etc. not to mention that i hate any windows other than XP, esp 64bits, some of my app cant be installed in it. but it seems inevitable on this 24GB RAM machine. and what is sucks more is the AMD only produce 64bits Radeon now

i dont know what they are thinking, i should be aware of this before buying the card. anyway i believe it will be mostly under the shelf unless the kids want to play games with it. fwiw...
a bit related: https://www.eevblog.com/forum/beginners/home-made-dvi-i-to-vga-converter-possible/msg2696928/#msg2696928