These "complex" probability questions should always be addressed in a similar manner. Decompose them in smaller subproblems and solve them separately.
This problem could be decomposed in finding these probabilities:
* probability that the inner center is in its place
* probability that the $8$ corner pieces are at the corner positions
* probability that each corner piece is correctly orientated
* probability that the $6$ center pieces are at the center positions
* probability that each center piece is correctly orientated
* probability that the $12$ edge pieces are at the edge positions
* probability that each edge piece is correctly orientated
If you find those, you can multiply all of them and get the final result.