The cost of a Camera and a ViewPort (or FrameBuffer)

What is the cost of having a camera with an associated viewport? Maybe I’d like to make many small screens in my app, does each one cost as a normal screen - for rendering time and such? Will my app get roughly 2 times slower when I have 2 times more little screens? If so, is it possible to somehow “disable” screens so that, say, only 4 are visible at a time, while others are “minimized” and requrie no updates?

It depends so much of what’s in these screens that it’s hard to give you a simple answer.
If the question is is there an overhead to have an aditional viewport and another camera, I’d say not that much.
If the question is, I want an additional viewport to render the same scene twice, I’d say it’ll probably cut your perf drastically. But the perf loss is more in “render the same scene twice”.