Gold costs go up quite a bit for purchasing post-industrial - having over 1k gold per turn means you can comfortably upgrade/purchase a unit or two every turn.
For context, in my recent China King game (playing Tradition), upgrades my frigates to cruisers cost 1500 Gold each. And I was making about ~600-700 GPT with 4 cities.
Since small tradition games tend to be more gold lite, this is a good lower end metric for what it looks like.