[martinmcnally]

That light cannot be dimmed using a controller without modification, thats why the cost is so low.

You would have to open the fixture and determine what kind of drivers are in there. You would need to find out the current output and voltage range (min and max).

You would then need to remove the drivers and replace with a dimmable drivers with the same current and voltage specs, plus some dimming circuitry or controller.

As a rough idea you are looking at at LEAST $200 for drivers that will produce the needed 300W

Unfortunately LED drivers cannot be dimmed using the power cord like some dimmable T5 ballasts can.