Increasing Research costs question

Hello I am trying to increase the research percent cost to make my game last longer. I can set the percent to 750% without any issues however when I set it any higher the Technologies in the later eras always have -research costs so they complete it 1 turn.

I am guessing its because I am missing some kind of option that removes a limit on how much research points a game can last but I cannot find the option anywhere in the Civ files. Anyone have any solutions to this problem?

Thanks