How is the percent-based gold production boost calculated?

Question

Many buildings in Civ 5 provide a percent-based gold production boost (eg. Market, Bank - +25% gold output).

The question: how is the bonus calculated?

Possible options:

1. Once, when the building is finished. This way, a city with gold output of 20 gets additional 5 gold per turn bonus that stays equal to 5 til the end of the game.
2. The bonus is recalculated as the game progresses, and cumulates additively. This way, the city always gets the bonus of 25%. If another building is created with the same 25% boost, the total boost is equal to 50%.
3. The bonus is recalculated as the game progresses, and cumulates multiplicatively. If another building is created with the same 25% boost, the total boost is equal to (1.25 * 1.25 = 1.5625, so 56.25% total boost).

Thank you for your time.

Answer 1

All percentage bonuses (not just gold) are based on the total raw output of the city.

So when the city makes 10 gold per turn from raw tile / building output, that 25% bonus gives you 2.5 gold. (You can never have fractional gold in your treasury, but if you have an additional source of fractional gold, it gets added together). When the city later makes 20 gold per turn (due to increased population, for instance), that same building provides a bonus of 5.0 gold instead!

So, "Option 2" in your proposed list is most akin to how the game works.