Is Classic Retro Arenas & Hazard Grid Pack free?

Yes. Classic Retro Arenas & Hazard Grid Pack is released under Open Source (GPL-3.0) and costs nothing. Neither it nor Mrboom has any paid content.

What do I need to run Classic Retro Arenas & Hazard Grid Pack?

Mrboom, which is itself free. Classic Retro Arenas & Hazard Grid Pack is an add-on and will not run standalone.

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

Roughly 15 MB, on top of the base Mrboom installation.

Yes. The source is published at github.com/Javanaise/mrboom-libretro under Open Source (GPL-3.0).

Classic Retro Arenas & Hazard Grid Pack — Free Mr. Boom Mod · PlayBound