The first criterion is not price but compatibility with your machine. A blue diode does not get through clear acrylic, does not mark bare metal and skims over clear PET-G: on those materials, no power value makes up for the wrong wavelength. So the comparator shows compatible sources first.
The second criterion is the thickness actually available. A material sold in 6 mm minimum is of no use to a 10 W diode, which would spend an unreasonable amount of machine time on it. Conversely, a 3 mm panel cuts in one pass and lets you test a batch without tying up the machine all day.
The third criterion is the type of operation: many materials engrave but do not cut — metals, glass, stone, Corian. Buying an aluminium plate to cut a shape out of it is a classic and costly mistake: a marking fibre does not cut metal.
Finally, always check a material is not forbidden before ordering. PVC, raw ABS, polycarbonate, HDPE, chrome-tanned leather, polyurethane foam: these damage the machine and release hazardous gases, and their guide states what to use instead for the same purpose.