I'll take a guess and say you have never been on track before... you don't need a cage. Join your local PCA and go to their High Speed Drivers Education (HPDE) events. I have been running HPDE for 10 years and still no need for a cage. If you are seriously concerned you could look into several of the well made bolt in Roll Bars. Very quality products that are removable with almost no evidence left in the car.
As a beginner, just go try it out! Who knows... you may not like it

Or you'll be stricken like the rest of us here... forever doomed to a life of waiting for the next track day! All you need to go to HPDE is a helmet and a fire extinguisher. Very small investment that you can liquidate if you find it is not for you. Though I would also recommend reading Skip Barbers "Going Faster" before you get there just so you can make the most out of what the instructor is teaching you.
Now, all that said, if you are really going racing you need a cage. You have to research or pay for knowledge on how the cage should be built just like Racer states above. Also, once you properly cage the car it will lose value to most buyers and you have shrunk the market down to track junkies for buyers.
In S. CA you will find a ton of help and great people. Pick their brains. You might even get one of them to give you a ride in some hot laps so you can see what fun it is
Just some thoughts
-Michael
P.S. My local cage costs for good custom welded in cages run 1750-3000 depending on various options and material. If you can wait, try and time it when steel prices are low
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Michael 
'82 Euro SC 'Track Rat' 22/29 Hollows, 22/22 Tarrets, Full ERPB F/R, Rennline Tri Brace, Glass bumpers, Pro 2000's, 5 pts, blah blah blah
'13 Cayenne GTS