Roblox occasionally runs referral programs or "Pls Donate" style events where you can earn small amounts of Robux for free (through Microsoft Rewards or Google Opinion Rewards converted to Roblox gift cards).
Since you cannot create a personal private server for 0 Robux, use these alternative methods: Roblox Rise Of Nations Free Private Server
If the host is in the server, a quick thank you in the chat goes a long way in keeping the link active for everyone. Summary: The Ultimate Sandbox Experience Roblox occasionally runs referral programs or "Pls Donate"