@morningstar: The price could be contingent on the amount of upgraded cards you have, for example, so a player with no upgraded cards would only have to pay 1500 (10% of price I suggested), and somebody with, say, a handful (5-6) of upgraded cards would have to pay 3000. Numbers are just to explain the idea, in no way concrete.
@Ash: My suggestion would simply allow players to get a second chance, so to speak, with their nymph. If they spun a color they don't want, they can change the color. Another option would be to allow players to spend a large amount of electrum to randomly change the color of one of their nymphs a day, excluding the colors of any other nymphs you have. This seems like a very practical idea to me, since you're in no way guaranteed a specific color nymph, only a different one (would always change the color, never "choose" the one it already is). A player exchange would let people hoard many nymphs of the same color and get huge advantages in pvp (a deck with 4-5 purple and black nymphs and quintessences would pwn the vast majority of decks out there).