Not having the departure tax included in the ticket price is one of the main complaints from Cook Islands tourists. If you compare how much taxes are included in the ticket price for a trip to RAR compared to other destinations, you'll see that that the amount included in the CI tickets is less than for other destinations.

Here are some examples of arrival & departure taxes/fees, airport fees, etc. for various destinations for roundtrips from AKL (comparison made 2009), and as you can see, the Cook Islands is somewhere in the middle:

Norfolk Island: NZ$152.40 (included in ticket price)
Rarotonga: NZ$55 (paid upon departure)
Apia, Samoa: ST$40 (NZ$22) (paid upon departure)
Tonga: NZ$43 (included in ticket price)
Niue: NZ$25 (paid upon departure)
San Francisco: NZ$78 (included in ticket price)
Nadi, Fiji: NZ$63 (included in ticket price)
Papeete: NZ$49 (included in ticket price)
Port Vila: NZ$47 (included in ticket price)
Vancouver: NZ$82 (included in ticket price)
Brisbane: NZ$129 (included in ticket price)
Cairns: NZ$121 (included in ticket price)
Noumea: NZ$58(included in ticket price)

(The amounts were calculated by taking the total taxes/fees for an AKL-XXX-AKL trip and deducting the taxes/fees charged at AKL)