Some of the various messages gained when building these upgrades:

We have upgraded our trading port. It cost a freaking ton of money to do it though. It better be worth the wait too. But it's not like we had something better to do, sitting around the seashore all day, counting rocks.

We have spent an enormous amount of time and money in upgrading this port. With the amount of silver we spent to upgrade this place, you could have bought a lot of soldiers. We hope you have made a wise decision, because if don't have enough men to defend this city, I'm not going to be the one to take the blame.

I thought spending that amount of money on these port improvements was daft, but it's finally finished. If some more money doesn't start flowing through those nice new port harbor gates, someone's going to be in big trouble. And it ain't going to be me.

Oh gods! When will the hubris of man end?! We have spent unholy amounts of gold on building this new port upgrade. I only hope our people are still alive by the time we should be expected to make up the money on this project. I'll probably be dead by then though. And my son will probably be paying off his gambling debts as a slave in some Roman leathermaking shop. Woe is me!
This might give a perspective on the costs involved.