Currency in Cocos Islands to Australian dollar (AUD A$). For 10 US dollars you can get 13.9 Australian dollars. For 100 US dollars you can get 139 Australian dollars. And the other way: For 10 Australian dollars you can get 7.77 US dollars.
And if the trip plans are simply for a given region of the world, then maybe it will come out cheaper than in Cocos Islands in nearby countries? See prices in: Christmas Island, Singapore, Malaysia, Indonesia, and Brunei.