PCPartPicker part list / Price breakdown by merchant

CPU: AMD Athlon X4 860K 3.7GHz Quad-Core Processor ($66.99 @ NCIX US)
CPU Cooler: Gelid Solutions CC-Siberian-01 51.9 CFM CPU Cooler ($8.98 @ OutletPC)
Motherboard: Asus A68HM-K Micro ATX FM2+ Motherboard ($37.99 @ SuperBiiz)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1866 Memory ($29.99 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($49.98 @ OutletPC)
Video Card: Asus GeForce GTX 950 2GB Video Card ($128.99 @ SuperBiiz)
Case: Rosewill FBM-01 MicroATX Mini Tower Case ($29.99 @ Amazon)
Power Supply: Thermaltake SMART 650W 80+ Bronze Certified ATX Power Supply ($54.99 @ Amazon)
Operating System: Microsoft Windows 10 Home OEM (64-bit) ($89.88 @ OutletPC)
Total: $497.78
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-12-17 09:49 EST-0500

This rig more than ensures a solid experience in MOBA and CS:GO.
Around $500 with an OS included.