The price of cards are determined by an automated liquidity protocol set by the Skyweaver Market, powered by Sequence Marketplace.

Here you can find out everything there is to know about how Market prices are calculated: https://www.skyweaver.net/news/market-overview.

UPDATE: As of April 2024, Horizon no longer provides initial liquidity for gold cards.

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.