Well, the lowest possible is:
Intel i3-4160 - around 80$
Asrock/Asus/Gigabyte H97 LGA1150 mATX motherboard - around 60$
G.Skill Ripjaws X or Corsair Vengeance 2x4Gb 1600/1833Mhz RAM - around 50$
GTX 750ti - around 130$
XFX, Seasonic or EVGA 450-550W psu - around 50-60$
NZXT Source 210 mid-tower- 40$
Western Digital 1Tb hdd - 45$
Windows 7 - 90$
approximate total: 545$ - 555$
EDIT: Here's a proper list
http://pcpartpicker.com/p/mkXkrH
Intel i3 - 4160: $108.95
ASRock H97M PRO4 Micro ATX : $69.98
G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1866 : $48.99
Western Digital Caviar Blue 1TB : $50.89
EVGA GeForce GTX 750 Ti 2GB: $129.99
NZXT Source 210 Elite (Black) ATX Mid Tower : $41.99
SeaSonic S12II 430W 80+ Bronze: $58.99
Microsoft Windows 7 Home Premium SP1 OEM (64-bit) : $89.91
Total: $599.69