It is currently impossible to make a private server cost a flat fee instead of a monthly subscription.
This flat fee would as a lifetime ticket; This would act the same as game passes, paid access, and even things like clothing or accessories.
As a developer you can add this for any reason you want.
Some players forget they had a private server and keep paying the developer their Robux. This is the only subscription service on Roblox except for Premium. It’s odd. (@Maxx_J confirms this happens.) While this won’t solve the problem, and ultimately it is up to the player… it will lessen the blow and prevent unintentional cheekiness.
Unlike developer products which you buy at your whim, subscriptions are automatic.
I recognize that flat fees don’t work for everyone, but neither do subscriptions. @Zeezy2204, for example, has their private servers grant powerful freedoms and they’d prefer these players to pay a little more.
Thanks for reading. Thanks also to @fireboltofdeath for helping me make this topic.