Bugreport:
The costs for a gift to the pirates or also other GiftBalancing related things, like asking them for ceasefire, do not change, regardless of the ProgressLevel of the human players (EarlyGame, LateGame and so on) (cease fire costs do change based on military strenght and reputation, but not progress). I'm a modder, but I confirmed this behaviour without any mods.
Why is this a problem:
In the earlygame when you don't have much money, the current costs are often a bit too expensive, while in the later game they are really really small compared to the money one has. I tried to fix this via a mod, but did not succeed, I fear this is a bug that Ubisoft has to fix.
Why this is a bug:
The costs of gifting does increase like it should for 2nd party AI players (so it is for sure also intended for pirates, as you can see in the assets.xml, it has "IncreasePerLevel" defined, while it is obviously not used).
From my observation it might be tied to the progress this AI player makes, not on the progress the human player makes (is this intended?). This might be an explanation why it does not change for the pirates, because they have no progress.
So a fix could be to make the pirates costs scale based on the humans progress and not on the pirates progress.
How to reproduce:
Start a game with pirates and one AI on fastforward , go to diplomatic menu and hover over the "make gift" button to see how much money the gift includes to the pirate and to the AI player. Then let some time pass, maybe build a small settlement with workers and then compare the gift amounts again. You will notice that the value from AI did increase, while from pirates it is still the same.
Personal opinion:
This is one of many small oversights existing from the first day unfortunately, Anno1800 was rushed and too much focus was put on ornament dlcs (they give more money, so it is an obvious decision) instead of completing game mechanics, balancing and bugfixing =/ Luckily mods can help, but they can not fix everything unfortunately...