Perhaps, maybe, might be, could be

These are largely interchangeable.

To the question: "Is there life on Mars?", for example, "Perhaps", "Maybe", "There could be" or "There might be" are all equivalent answers.

To the question: "Are you going to the cinema tomorrow night?", "Perhaps" and "Maybe" are equally valid answers (indicate the person is undecided). "I might be" suggests the answer is dependent on something else (e.g. "I might be, if my sister doesn't visit"). I don't think you would answer: "I could be", in response to this question. The difference is very subtle, but "could" seems to relate slightly more to whether such a thing is possible, not whether it is likely. Obviously, I "could" go to the cinema (nothing physically prevents me). But if someone is asking about WHETHER I'm going, "I might be" is the better answer.