Is Surf Utopia CS2 free?

Yes. Surf Utopia CS2 is released under Community / Steam Workshop and costs nothing. Neither it nor Counter-Strike 2 has any paid content.

What do I need to run Surf Utopia CS2?

Counter-Strike 2, which is itself free. Surf Utopia CS2 is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from Counter-Strike 2's mods list. You can also install it by hand — full steps are on this page.

Roughly 40 MB, on top of the base Counter-Strike 2 installation.

Surf Utopia CS2 — Free Counter-Strike 2 Mod · PlayBound