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Depending on the developer, and the scale of their game, these things can also be incredible cheap to produce too. If your gameplay/monetary loop is something designed to arbitrarily force a player to wait to accomplish something or otherwise spend money, then you can drastically reduce the amount of content that needs to be added as long as you have an adequate base.

Even if you spend money, loot box mechanics and randomized stats can push players to continue to spend because while they got an item, they didn't get the perfect item. Base builders, team combat titles and character based games are very, very effective at this.

For developers like the one behind Evony, they can be a lot cheaper because that game, and a hundreds like it have existed all the way back as far as farmville and earlier. They just got better at the monetization loop over time.