We're in the process of replacing a key for our VW Jetta, so I've gotten to be fairly well versed on the stupid keys with chips
A lot depends on the system used for your car. IF the thing has an immobilizer in the system, and you also have a remote opening/locking keyfob...you have 3 problems to solve.
1)finding the correct key fob. With our VW, I can program the keyfob (for locking/unlocking/trunk) if I have 2 physical keys. (it's not hard, just follow strange directions..hold button while turning to accessories, hit button 2 times, etc.).
The actual KEY that turns the ignitions has a chip in it that disengages the immobilizer. An immobilizer is a computer chip implanted in the ignition system that sends out a signal to the key chip "hello? are you there?". If it receives an answer that it understands, it allow the engine to continue running. If it DOESN'T receive an answer...it shuts off the engine and locks it.
3rd problem for my car is that the key is laser cut on the inside of the blank, not the outside edges. Takes an $8000 machine to do the cutting. So only auto dealers, and now some locksmiths have the machine.
To reprogram or newly program the keys, you need a specific code. The courts forced the car companies to make those codes available to locksmiths slightly over a year ago.
For the VW cars (and audis) you can buy a keyfob from Ebay (or wherever), reprogram the locking mechanism but not have a physical key for that keyfob. You can purchase a blank physical key and have it cut, and then take it to your dealer to have the chip programmed/replaced. (there are some locksmiths that can do it, too...you just have to find them). OR you can buy the whole thing from the dealer.
Our dealer charges $85 for the keyfob, $100 for the key and programming. I bought a keyfob on Ebay that matches our keyfob's ID numbers for $15. The blank will cost $15. Cutting and chip will be $50 wherever I go.