A Smarter Mirror for Cheaper Solar Power

Mirror mirror on the wall, whose the cheapest producer of all?

It would seem to be mylar...

http://www.appropedia.org/Reflecting_materials

whose who's

Hu's Mylar?

whose Determiner (relative possessive) /ho͞oz/

1. Of whom or which (used to indicate that the following noun belongs to or is associated with the person or thing mentioned in the previous clause)

In this usage, reference is made to mirror(as in reflecting surface efficiency)....The reflective quality plus cost of unit per square ft, equals best initial value....added to this would be longevity, or cost of coating materials and/or treatment that would produce best results...Your assumption that the reference was a direct correlation with quote from "Sleeping Beauty", would be incorrect in that the rhyme used in it's original context was referring to the "evil witch" whose reflection is the subject of the inquiry....I rest my case