From Liquipedia Warcraft Wiki

Upkeep is the taxation of gold income. The level of upkeep depends on the player's population.
Despite the fact that gold income from Creeping, selling items and Transmute is displayed by a yellow floating text, that does ignore upkeep, gold income from these sources is also reduced by upkeep.

Upkeep Levels[edit]

No Upkeep[edit]

  • 0 - 50
  • 100% income

Low Upkeep[edit]

  • 51 - 80
  • 70% income

High Upkeep[edit]

  • 81+
  • 40% income