Surely it costs more than the cardboard, it's not like egg cartons are made from quality board, it's reconstituted 'crap' that's just enough to hold together supporting a dozen eggs.
OP here. Yeah, I don't know why either, but it seems most eggs in my area come in these plastic/styrofoam cartons. Not in 2 packs like my post though, I cut that out of what was an 18 pack, as a wallet to protect my money, obviously.