City Tier / Level Increase?

[Elanor]

Yes, 1.8 million gems:

@Night_Guy

• By extrapolating, the purple box reaches its maximum potential at an expenditure of 225,000 gems on buildings/momuments, while the yellow box does so at 450,000 gems, the blue box at 900,000 gems, and the brown box at 1.8 million gems.

Attached is a table to convert your building tier to gem value: