Depending on when you want to travel and how far in advance you book your tickets, travel between the Hawaiian islands can be very reasonable. Flights can be found as cheap as $39 each way on Hawaiian and Southwest Airlines. $39 is the best we've seen. Expect to pay between $50 to $100 for a one-way ticket.
What island in Hawaii are tourists not allowed?
Once owned by Royalty, Niihau was purchased from King Kamehameha in 1864 and up to 1987, visits to the island were typically restricted to the owners and their guests, or government officials, which is how it gained its moniker; “The Forbidden Island”.