Maximum spending per turn / minimum turns to unlock for technologies

I know at least one other game (FreeOrion maybe?) does something like this. Would prevent rushing the tech tree when you have lots of RP. Would also synergize well with a leaky research mod where you can get random breakthroughs depending on your progress in a tech!

As for which way to do it, I'm leaning toward maximum spending per turn... the modder has to do a little math but then the player can spend his extra RP right away on other techs!