A lot less than you might think - something like this
https://www.ebay.co.uk/itm/HP-DL380-G7-2U-RACK-SERVER-2x-E5620-48GB-P410i-256MB-3TB-STORAGE-2x-PSU/253970907009?epid=222672092&hash=item3b21d87f81:g:Oa8AAOSwxOFaWMil
or
https://www.ebay.co.uk/itm/HP-DL380-G6-16-Bay-2x-Six-Core-Xeon-48GB-RAM-P410-FBWC-ProLiant-2U-Rack-Server/143158078828?hash=item2154e2ed6c:m:mos8_vGlLJDU6OnvPgwZanA
more than 2 processors and costs can go up
but not always
https://www.ebay.co.uk/itm/IBM-X3850-X5-Server-4-x-E7-4830-8-Core-CPU-32-cores-2-13Ghz-128gb-Ram-No-HDD/372642294000?hash=item56c33628f0:g:0tAAAOSwEfVcUZ1z