Also have a look in the gallery on that site i linked above, has some different designed cages to give you a few ideas.
Cams approved cage bent up for a series 6 RX7 cost about $900 for material and all bends and cuts, then you need to weld it all in ... it might be cheaper for other cars due to the rx7 being a complicated design. that was from a places in Mc Graths Hill NSW