Cheap Minecraft server hosting for 5 friends?

If you all play on PC with mods, there's also the Essential mod. It lets you open your singleplayer world to friends straight from the game, no port forwarding. The world is only up while you're on though, so it's not a 24/7 server.

For 5 people you don't need much. Vanilla is happy on 2-3GB, modded wants more like 4-6. If one of you has a decent PC that can stay on, hosting it yourself is free and usually runs best. You just need to port forward, or use a tunnel service if your ISP blocks that. Otherwise most cheap paid hosts are fine for 5 people. Pick one with a server location near you and skip anything selling "unlimited" slots. Aternos is free but you wait in a queue every time you start the server.