You might be able to exchange your miles with people who have thankyou points and book MOZ-BOB with thankyou points.

In case you have to purchase tickets out of pocket the cheapest I have found was by getting Citi Prestige card. You pay $400 fee and use BOGO to get two tickets (adding a AU for $50 more you get 4 tickets for the price of 2 which was what I did). You can even book all the way to BOB to apply BOGO on the inter-island flights.