Well at the moment we have more of an increasing supply. The new reward cards just began to be printed. They're at about 2% printed right now. It will take about 100 more days even with an increasing amount of players. And with the same increase amount in player base, there will be an increasing amount of cards cause they can also loot them and might sell cards they don't like.
All in all the price of all reward cards is still on a down trend on a day to day basis and will hit probably 3-8 cent at the end.
So buying is totally fine. I myself bought a bunch of Nightmare and Charlok Minotaur but I'm still a little cautious about the new cards. Otherwise, they may take a long time to rise in price above the purchase price.