Linuxcnc's Plasmac config supports RS485 out of the box.
Docs here: PlasmaC User Guide
More plasma info here: Plasma Cutting Primer for LinuxCNC Users (full disclosure, I wrote this)
Linuxcnc is open source (free) and for a kickass build you could use a Mesa 7i76e plus a Mesa THCAD-5 (for 50:1) or THCAD-10 (change the board to output 30:1) which will set you back about $250
The cheaper Mesa 7i96 and THCAD-5 will be < $200
You need a DB9 serial port on your PC and a RS485 converter. USB serial ports are not that reliable.
I've recently bought an Odroid H2+ PC which has a UART on the GPO pins so I hope it can be used for RS485
If you have moe questions, Ask in the Linuxcnc forum as you will get more help than here.