CPU: AMD A10-7850K 3.7GHz Quad-Core Processor ($129.99 @ Amazon)
Motherboard: Biostar A58MD Ver. 6.x Micro ATX FM2+ Motherboard ($41.36 @ Mwave)
Memory: G.Skill NS Series 4GB (1 x 4GB) DDR3-1600 Memory ($36.99 @ Newegg)
Storage: Toshiba Product Series
T01ACA 500GB 3.5" 7200RPM Internal Hard Drive ($41.99 @ NCIX US)
Case: Rosewill REDBONE U3 ATX Mid Tower Case ($24.99 @ Newegg)
Power Supply: Corsair Builder 430W 80+ Bronze Certified ATX Power Supply ($19.99 @ Newegg)
Optical Drive: Lite-On iHAS124-14 DVD/CD Writer ($12.99 @ Newegg)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Wireless Network Adapter: TP-Link TL-WN725N 802.11b/g/n USB 2.0 Wi-Fi Adapter ($8.07 @ Amazon)
Total: $406.35
For Minecraft, you only really need a 250 GB hard drive but you never know. The network adapter is so you can plug it into the PC so you can play via wifi and not ethernet.
(Should run minecraft 1.7.10 at around 40-80 fps on decent render distances)