If you dont like the jumpy price guides, id say if some one is willing to buy it (hince ebay members) they would be up to and excess of $15. Just my opinion, i say a llanowar wastes foil on there for $16.50.
I would sure set the price pretty high... the cards are worth enough alone... foil makes them $20-25 easy(for someone who wants them)...
but I am also someone who hates to buy singles so I really have no idea...