Cheap refurbished desktops with quad core processors starting at $119.20 similar to the deal from Scrimshaw (https://www.ozbargain.com.au/node/340269).
First PC is Dell Optiplex 980 SFF
i5 650 3.2GHz (2C/4T) (Geekbench Single-Core Score 2360 Multi-Core Score 4640)
4GB RAM
320GB HDD
$150.39 shipped
http://www.ebay.com.au/itm/Dell-Optiplex-980-Desktop-i5-650-…
Second PC is HP Compaq 8200 Elite SFF
Core i5 2400 3.1GHz (4C/4T) (Geekbench Single-Core Score 2446 Multi-Core Score 7991)
4GB RAM
250GB HDD
$199.20 shipped
http://www.ebay.com.au/itm/Refurbished-HP-Compaq-8200-Elite-…
Third PC is HP Compaq 8300 Elite SFF
i5-3470 3.2Ghz (4C/4T) (Geekbench Single-Core Score 3452 Multi-Core Score 9626)
4GB RAM
500GB HDD
$229.59 shipped
http://www.ebay.com.au/itm/HP-Compaq-8300-Elite-SFF-i5-3470-…
The first desktop, with the Core i5 650 is first generation. I probably wouldn't bother listing that one as that's a very old pc (from 2008~09)
The best value one is the HP 8300 Elite. I would move that to the first item in your search parameters.