Prey has been featured as a "Free Game of the Week" on the Epic Games Store in the past. It is worth checking their weekly rotations to see if it returns.
The full version of Prey (2017) is frequently included in the Xbox Game Pass for PC library. This is the most cost-effective legal way to play. prey-free-download-pc-game-full-version
If you prefer owning your games without digital rights management, GOG.com frequently hosts sales for the digital deluxe version. Game Overview & System Requirements Prey has been featured as a "Free Game
During major seasonal events, the game often sees discounts of up to 80-90% on Steam . prey-free-download-pc-game-full-version