So, I've been messing around some games trying to figure out how GP cost progresses. Turned out it's much simpler than one might assume.
All GP has a fixed cost per era. It only depends on the era of that GP, so all Modern Era GP cost the same, be it Artist, General or Scientist. There's only one factor that might change its cost.
There's actually only one thing that changes the cost.
If the world's average era is below of the first GP of a new era of a certain type, there's a fixed increase in price. This increase is only applied on the first GP from that type of that era.
For example, if you recruited all Great Merchants of the Renaissance Era, then the next will be of the Industrial Era (say, Adam Smith). If the world is still in the Renaissance Era, the increase will be applied for him, but not for the other 2 Great Merchants from Industrial. It'll be reapplied for the first Great merchant of the Modern Era again if the world didn't reach Modern in the mean time.
Remember the increase is applied for each type separately; in our example, having recruited the first Great Merchant from Industrial won't help with the increase of the first Great Engineer from Industrial.
Those costs are (the numbers in the parenthesis are the increased cost):
- Classical Era: 60 (??)
- Medieval Era: 120 (??)
- Renaissance Era: 240 (310)
- Industrial Era: 420 (545)
- Modern Era: 660 (855)
- Atomic Era: 960 (1245)
- Information Era: 1320 (1715)
Some notes besides the actual cost:
- All games have by default all Great People available.
- If the world's era advances farther than GP era, then all GP from previous eras are forfeit, except the current one (the one that appears when you open the GP window). After you recruit the current GP, the next one will be the first of the world's era. That's notable as some AIs beeline Cartography, bypassing the Medieval Era altogether and may result in all Medieval Era GP being lost.
- How the game calculates the current world's era is still largely a mystery to me. I suspect it checks where the most advanced tech/civic of every player lie in the tech tree, then it checks which era at least 50% of the civs have passed. If a player is in the second half of a certain era, it counts as the next one. For example, if Apprenticeship is the most advanced tech you have, it'd count you as being in Medieval Era; if you research Education, it'd count as Renaissance Era. This is still mostly speculation, though, as the way I checked the world's era was how the Great People from previous eras disappeared (also, it'd be strange for the world to be in Atomic Era when even I hadn't reached it).
Any inputs, especially on how the game calculates the world's era, are greatly appreciated.