Wow, I was idling on Storm and got 2 drops of the same card, one regular and one foil (Autumn). The regular one is going for $0.98 in the Steam Market, but I can't seem to find any of the foil for sale. So I take that as meaning mine would be the only one in the market.
Checking some prices, it seems there's only another one foil card of this game being sold (different one), and it costs $15.20. So I don't really know how much to ask for mine, maybe $15?
EDIT: fuck it, I put it up for $20. If someone buys it, then great. If not, then I wouldn't have lost anything, and I can always put it cheaper.