I think it should be based on the card you draw, basically you win the cost of the card in Electrum.
Maybe if you want to be really nice to us, have a 4% chance of drawing an upgraded card (and winning the cost of the upgraded card), and let all cards -- including nymphs -- have the same chance of being drawn. Seriously, there are nearly 150 cards (not counting upgrades) in the game, so even if you have 12 Nymphs in that 150, it's less than 10% that you'll get a Nymph, much less a specific Nymph.
Or, if you want them to be ultrarare, combine the ideas of the upgraded card at 4% with the all-cards-at-equal probability, but you only actually win a Nymph if you draw the upgraded version. That's a .4% chance per day of winning a Nymph at all, much less the one you're actually hoping for. At that rate, it would take 125 days -- a third of a year -- to be statistically likely to have won a single Nymph.