[Vanilla] The Mechanism of Great People

The cost of a Great Person will be even higher if it's more than one Era over the threshold (of World Era + 1).

For Renaissance, the cost is 240 normal, 310 if one Era over, and 610 if two Eras over.
For Modern, the cost is 660 normal, 855 if one Era over, and 1685 if two Eras over.
For Atomic, the cost is 1245 if one Era over, and 2455 if two Eras over.

The Modern case first happened to me in a Deity game, where of course the AIs went for Scientists, but also Patronage passed for double Scientist points. After all the Industrial Scientists were taken, Einstein cost 1685.

Also, it's incorrect that only the first GP of a given era-group is extra expensive. In my current [GS] game, the cost of Modern Great General John Monash was 855. When it was taken, the cost of Modern Great General Samori Touré was 855.

And after that last Modern General was taken, the cost jumped to 2455 for the first Atomic General, Douglas MacArthur.
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