Tech costs adjust twice during the game (techs become more expensive during what is supposed to be the classical era and then revert to their original costs again at some point). This is a side effect of the extended classical era that Rhye introduced. Now that I have fixed the era tech costs factor it could be possible to remove this effect, because I agree that it is unexpected and encourages strange strategies. A while ago, there was some discussion about factors influencing tech costs, and I still have a ticket about displaying these factors better in my to do list. When I get around to that, I will also split these sorts of effects to factors influencing tech costs and factors influencing your research output, depending on what makes more sense. In any case it will then be documented and displayed properly.