We deliberately made the efficiency of exchanging Cubits for Plots constant so that there was no encouragement to save your Cubits for bigger exchanges in the future. If it was better value to exchange for larger Plot Coffers, then players would feel encouraged and even forced into grinding levels before exchanging. Given the game is often critisized for being grindy we wanted to avoid this.
If you want some plots you can take them at any point and not be concerned that you’re losing value.
There will be an efficiency in buying larger Cubit Coffers for real money. So there is some scaling in the entire system.