To build a artisan quarter allways cost 25 culture. 

On normal, well, thats normal every 10th turn, on endless its realy cheap every 20th.

If it cost 25 on normal it should cost 50 culture on endless.

But what if you allready have an extractor?

Since the more buyers you have the more gold you get I compare one without trade partners.

On normal every 10th turn you get 20 gold for 25 influence.

On endless, every 20th turn you get 20 gold for 25 influence.

 Endless SHOULD give 40 gold for 50 influence.