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The cost goes up the further into the game you are, and you may need to add a little gold to the deal if you are far enough ahead of the civ you're making it with. Upon completion, you both get a science boost equal to 50% of the median cost of all techs that you (and only you) can currently research. Scientific Revolution in the Rationalism policy tree and The Porcelain Tower both increase the boost by 50%. They stack additively, so with both you get a 100% increase.