If one gets on a taxi from a hotel to the AE station, it's extremely common for the cabbie to offer you a discounted price to bring you directly to the airport. In fact, if the driver doesn't suggest that, either he/she's a fool or he/she's ending his/her shift soon and doesn't want to go to the airprot.

Instead of $300 or so from Kowloon to airport, $200-220 is a reasonable offer. For two people it's about the same price as the cab fare to the AE station and two regular AE tickets. You save the transfer from cab to train, and it takes about the same time.