It isn’t clear to me why you are converting to mm if the price is in meters^2. The width and height inputs are in meters, correct?
Well, assuming that ((width / 1000) * (height / 1000)) gives you sqm, you can get what you want by using the IF function!
IF(((width / 1000) * (height / 1000)) < .5,((width / 1000) * (height / 1000)) * (thickness / 2) * number_plates,((width / 1000) * (height / 1000)) * thickness * number_plates)
Is that what you are looking for?
