Depends, what do you want from it? A Dual or Quad Xeon Server from many moons ago probably wouldn't be crazy expensive. Problem is there's not much you can do with them that a low cost modern CPU can't do better. And if memory serves Windows is locked to allowing a single CPU apart frrom the Pro versions.
Buying fancypants used server / workstation mobos usually simply isn't worth it, even if you have an actual use for it. For example: https://www.ebay.co.uk/itm/IBM-Leno...2011-FRU-03T6735-03T6501-03T8422/273140143398 vs: https://www.ebay.co.uk/itm/Lenovo-D...DR3-2x500GB-Quadro-K4000-CAD-VFX/352795706778 Yep, thats the mobo alone costing like 50% as much as a whole system built around it.