As you have bigger and bigger armies the maintenance cost increases, Base Maintenance cost is when you don't have any extra maintenance cost due to army size. And Maintenance cost shows how much it will cost to maintain that unit with the increased maintenance from the army size.

I am working on a vault change that is less harsh but also on a way to reduce production on the top end. Multiple people generating 200M a claim is a bit problematic and the change was aimed to reduce their claim amounts without affecting other low level players overtly.