These are thin films and the material cost is going to be relatively low. From the paper: >We found that platinum was ideal as broadband back-reflector, while

These are thin films and the material cost is going to be relatively low. From the paper: >We found that platinum was ideal as broadband back-reflector, while 20 nm gold was still ideal color-wise for the semitransparent nanohole layer. A one square meter sheet that's 20nm thick would require about .02 cubic centimeters of material to cover. For gold that's ~.4g or $23 worth at the price you've listed.