There are 4 different ways a card can be considered premium: Foil overlay, Gold Borders, Plat Borders (Preorder only), and Infinite Borders (competitive play only). Note that cards can be Goldfoil, Platfoil, and hypothetically InfiniteFoil.
While a premium card is certainly worth more than its vanilla version, it's highly dependent upon how good the card itself is--and its rarity. A Gold Lucca or Coyle, for instance, is worth much more than a Gold Stumble.
With that said, Demon of Solitude and Flashbang aren't the most-played of cards, so even in foil they're probably not very valuable. A draft deck is probably not worth buying off of those cards alone.