Railroads and Forts....

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Iirc the problem is that money paid for the buildings vanishes from the game, so if you make them too costly, you risk draining the global economy.
Goods wise the problem is that railroads have no upkeep, so if you increase the good cost for construction you get giant demand spikes whenever a big nation reaches the next level, causing a bubble for all related industries that crashes HARD once constructions are done.

 

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Does that mean the POPs supplying the goods for construction don't get paid?

No, the goods are bought and paid as normal, but some buildings (forts and ports?) have extra money cost.

 

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What if we had to garrison forts by putting infantry in them? Since the AI already has to manage troops, it might go over better with them. Perhaps you could even get a discount to troops sitting in forts.

Think of it this way: This is already absracted by the increase in siege time. Plus: Unlike EU or CK, there is no assault mechanic anyway.
It would have been nice, though, if buildings like forts or railroads had some sort of upkeep cost. That would influence demand on the world market, reduce the spikes from construction and make it less optimal to build stuff everywhere.
...well, let's wait for Vicky 3.