Here's how Cost works.
Each Era has a specific cost (60 for Classical, 120 for Medieval),
When you recruit a person and the game is in the same Era, the cost doesn't change. If you recruit a person but the next person is from the subsequent era, the cost is inflated (120 becomes 155).
Problem *is*, when the game Enters Medieval era, and said Person is yet to be recrutied, if you recruit era the next Person is still from Medieval Era, and once against Costs 120.
What *then* happens is that the price goes down while other player's points don't, so if you recruited it naturally at 155/155, but Player 2 had 130/155, then suddenly they find themselves at 130/120, they recruit, the next player has 120/120, and all of a sudden the price inflates because all of the same Era's people have been recruited, this is why you often observe Great Scientist being taken like hot-cakes at a baking sale.
It's one of my bigger gripes with the game. I also think if by some strange miracle You are recruiting a person from an older era, the cost is cheaper.
As far as I know, Recruiting with Gold and Faith has zero effect on the cost of the next person, only if you pass does the cost go down.
Also, the price inflation varies with era difference, If the game is Medieval Era but suddenly you are recruiting Industrial Era, the cost can be as high 1250, which makes the situation even more ridicolous.