Is it some kind of edible silver foil? I have no idea - wouldn't even know where to start?
You build the cost of the silver leaf into your pricing. That's if you have a business!
And gold leaf is more than twice as expensive :-) I do quite a few of this style of cake, a style inspired by Faye Cahill. Google "Faye Cahill Images" and you'll see tonnes of silver and gold leaf cake designs.