I found a good cage builder who works from a shed on his property about an hour and a half out of Brisbane to build my cage for significantly less than most of the race fab shops. I paid $1450 for my cage with fabricated drivers side seat mounts. You would expect to pay at least $1000 more anywhere else. I think the material costs were around $600, and it took about 15hrs labour.

Unfortunately building (and running) a race car costs money. I wish you luck, but I don't think anyone can put a car on the track for under 3 grand.