If you wanted this to work either as a submod or whatever you would need to make a building chain to accurately represent the cost. I have no idea how long a building chain can be but make it as long as possible and have each stage be quite short. Such as a 1 turn, 20 000mnai step that needs to be built half a dozen times or so until the restoration is complete. Maybe more if possible. Rather than a single 20 turn building at 20 times the cost. This allows gradual restorations when funds permit rather than just saving 500,000mnai or only restoring it when you have conquered half the world and have a massive income.

This would actually be nice and would add a bit of flavour to the game, little personal challenges you could work towards, and having to make the choice to cut back spending in order to finance a bunch of buildings. As it is, once you get past a critical mass this game just becomes unloseable and needs a bit of roleplay to keep it reasonable.