The price of cards are determined by an automated liquidity protocol set by the Skyweaver Market, which is ultimately based on Niftyswap.

You can find out everything there is to know about how the Market price is calculated: https://www.skyweaver.net/news/market-overview.

At the very beginning, Horizon will provide a small amount of liquidity to the Market for cards. As players start buying and selling, prices will be automatically determined through the protocol.