We use the tiered system which is a vanilla setting. Facepunch offers two types of upkeep: flat rate or tiered. We use their tiered system in an attempt to keep building sizes under control. Our server provides an extreme amount of resources and players cannot destroy your base which leads to overzealous builds that can cause massive performance hits on other players as they try to render in your base.

Basically as your base increases in size it can put you in a higher "tax bracket" so to say. There are 4 brackets (These are made up numbers and not the actual configuration as we do not divulge our configs to the public):

Each tier has a different "upkeep cost per piece".

Again, this is still vanilla and not some plugin or customization. It is just one of two ways Facepunch programmed upkeep into this game. And is quite common on PVE servers or increased gather servers to help curb insane sizes of builds (which even with our tiered system is our biggest problem from our players).