We had a Commuter and it was $874 incl 3rd party 2 years ago. When we reduced the seating in our Commuter from 12 to 3, rego cost reduced to under $700.
Yours would depend on whether it has all the original seats and if not it will also need a 'mod' plate for the reduced seating. If still registered in another state you will need a current safery certificate and a gas certificate if gas is installed.
A blue 'mod' plate is issued after a licensed engineer checks what changes you've made to the vehicle.
You can also phone Qld Transport and talk to a human...they're usually very helpful.
Good Luck