Source for cheap PLC/Software

Any recomendations on cheap, low end PLC's and software for a CNC
machine? Automation Direct has a nice $100 one it's but software costs
another $150. PLC can be bare board or cased, 8 input/outputs is fine.
The machine can provide regulated 5 and 24 DC volts, or AC for power.
PLC needs to talk some flavor of modbus.

Roger