How much does a Minecraft server cost?

Only up to a point. Metered bills every active GB-hour, so it wins whenever your server sleeps for real stretches of the day — but past roughly 8.3 hours/day awake (any RAM size — the crossover doesn't depend on GB), you've paid for the equivalent of a full month anyway, and flat becomes the cheaper, simpler choice. The 24/7 scenario above shows exactly that case.

For vanilla or Paper with plugins, 2-4 GB comfortably runs a small survival world or SMP with friends; step up toward 8 GB for a busier plugin-heavy SMP. Modded servers (Forge/Fabric) start at 4 GB, and heavier kitchen-sink packs typically run best at 8-16 GB — there's no single number that fits every world, so start where your budget fits and watch the tick — TPS and MSPT are live in the dashboard on Forge and NeoForge packs, and on Fabric packs from Minecraft 1.20.3 (on Fabric packs we count the world's own tick counter, so the rate you see is the one it actually delivered; older Fabric packs have no tick command at all, and the panel says so). You can resize later from the panel (self-serve up to 16 GB; 24/32 GB on request).

Yes — 20 active hours a month on a 2 GB server, no card required. It's bounded, not unlimited: free servers are limited per region (live availability shown on /free), and mods need at least 4 GB, which isn't available on the free tier.