Howdy,
club level motor sport such as hillclimbs and circuit sprints are your cheapest and most value for money style of racing for a new starter.

CAMS is the governing body for most motor sport in Australia.

Firstly you will need to join a CAMS affiliated club.
The NDCC (nissan datsun car club)of SA is a good option as many of it's members are into motor sport and can help you along the way.

The MSCA (marque sports car association) run circuit sprints at Mallala and you would need to be a member of the MSCA and an affiliated club to enter these events.

So thats $50 for the NDCC membership
$20 for the MSCA membership
$95 for a CAMS LS2 licence
(they all last for 12 months)

with these club memberships I did 14 events in 2008 - some were hiiclimbs and some were circuit sprints.

Great value for money, and you can use your street car if you wanted to.

Hope this helps.

Cheers,

Geoff