Hi, here's a short write-up on how to get a new key and fob for your C5 A6 for less than $50 (not including software/interfaces). I had a tremendous time trying to figure this out and hope this helps others.

1. Get the right tools: VAG Commander + cable (I used v1.4) and VCDS + cable (I used the latest version, but other versions should work).
2. Get the right parts:
2a. Split your fob in half using a small screwdriver inserted into the middle of fob. Use the first code you see to buy a matching fob off eBay. $20.
2b. Buy an Audi "flipkeyblade" with ID48 transponder on eBay from the guy in the UK. You'll have to provide him a photo of your key. $25.
2c. Take the eBay fob apart by using a screwdriver to separate it in half, remove the Audi symbol, unscrew the screw, take out the old ID48 chip and replace it with the new chip, remove the old key blade by pushing the tiny retaining pin out (don't push it out all the way!), put the new cut keyblade in, reassemble.
3. Plug in and use VAG Commander to get the SKC:
3a. Target: 07 Kombi
3b. Open channel (I had to use the K connection type)
3c. Instrument/Immobilizer
3d. Click read ... (this may look different across different versions of VAG Commander)
3e. The SKC will be listed as Security Access Code or as Code. Write it down.
4. Disconnect VAG Commander and connect VCDS to add the key:
4a. Hit Select, Go to 17 Instruments
4b. Login using the SKC recorded from VAG Commander adding an 0 in front (0xxxx).
4c. Adaption
4d. Enter 21 in channel number
4e. Read
4f. The "Stored Value" is the number of keys currently matched. Enter "new value" of 0.
4g. Save. This clears out the old key-matchings.
4h. Enter a "new value" corresponding to the total number of keys to be matched, including any existing keys. Don't forget the valet key.
4i. Save
4j. Done, Go Back
4k. Close Controller, Go Back - 06
4l. Switch ignition off and remove key
4m. Insert next key to ignition and switch ignition on again. The warning light for the Immobilizer will light up for ~2 seconds, then go out.
4n. Switch ignition off and remove key. Repeat steps 4m and 4n with each key until all keys have been matched.
4o. Switch ignition on. Now to match the fobs.
4p. Hit Select, go to 35 Cent. Locking.
4q. Adaption
4r. Enter 21 into channel and Read
4s. At this point my VCDS was different than the RossTech Wiki. The latest VCDS told me what to do: Enter value 1 and click Save. Then press unlock or lock on the remote. Continue to enter values 2 through 4 (depending on how many remotes you have) press Save, and press the unlock or lock buttons the the subsequent remotes until you have programmed all remotes.
4t. Turn the ignition off.
4u. Close out and disconnect VCDS.
4v. Test everything.

 

I need to try this, but maybe it should be stickied?

 

" not including software/interfaces "

Which happens to be specialist equipment which cost me as an auto locksmith thousands of pounds (or even more dollars).

so don't expect $50 for a key