Inkalu
Chieftain
techs get progressively easier to research as they spread among civs
Technological evolution. But I would change it a bit with a concept called neighboring bonus (from Europa Universalis II). If players neighbor owns a tech which he doesn't own, the tech should be cheaper to aquire. With techs and wonders suchs as writing, map making, steam power, flight and internet more and more countires would receive neighboring bonus. The larger the technological gap between countries, the larger the neighboring bonus.
In addition if the more technological country trades with less developed countries, they should have a discount to all techs known by the tech-leader.
-Inkalu