If we use the current nuclear power plant as an example. Its a +50% bonus to production. That is an amazing amount of hammers, and worthy of late game building. Or the spaceship factory, provides a +25% bonus to science, in addition to its production bonuses. Again, a really solid bonus. So if we don't go with instant bonuses, than number like above are what you need to make the buildings attractive. And in those numbers would be "too much", than we don't need more buildings. So that's my benchmark for late game buildings.