Given the time spent hunting for “free” wallpapers, the cost is negligible for most users.
If you prefer a completely free experience from the start, these open-source and free engines can run many of the same file types: wallpaper engine free wallpapers
A major concern with animated backgrounds is hardware drain. However, Wallpaper Engine is highly optimized: Wallpaper Engine on Steam Given the time spent hunting for “free” wallpapers,
Many third-party sites (e.g., wallpaper engine free .pkg files, or repack sites) claim to offer free wallpapers without the Steam app. These are almost always risky: wallpaper engine free .pkg files
Do not pay for individual wallpapers. Use the free Steam Workshop library included with the software, and be extremely vigilant against malware when downloading "free" wallpapers from unverified third-party websites.