Oceania (Australia, New Zealand & the South Pacific) - Brisbane airport early morning transfers

Can anyone recommend a cheap-ish way of getting to BNE for a 6am flight? I won't be staying in the CBD, so the likely option seems to be a taxi, which I've estimated at about $80 from the Eight Mile Plains area. However, a few people have mentioned that privately booked cars/limousins are often cheaper than taxis.

Is that the case? Any recommendations?

Air train first service dep Roma St at 5:04, Dom terminal at 5:34.
Check out www.coachtrans.com.au or www.con-x-ion.com