I'd expect fares to be higher than $100 for such an air shuttle.

Check out what they charge for North Shore to Whangarei (similar length flight but added cost for the shuttle into city and reduced cost as Dairy Flat would be cheaper airport than Mangere).

Then look at the schedule. With the North Shore Whangarei vv flights the a/c and pilot is well utilised, whereas the OP suggestion either leaves both a/c and pilot at Mangere all day or requires another flight out with either low or no loads - the costs of which need to be borne by passengers.