I lost my spare golf key and was quoted in excess of £200 for a new one from VW, so I shopped around. I thought I'd share this experience to save you all the hassle I went though finding someone to make the key that I bought useable!
I bought the key for my VW (actually not from eBay, but there are plenty on here).
I paid £40 for 2 button flip keyfob with uncut key blade (mine was brand new and uncut, but used is OK too, you just need a new blade that most auto locksmiths carry).
If you are down to your last key and need a spare, you need to do the following:
Split the existing key in half (don't worry, it snaps back together again) - the bit with the blade detatches from the bit with the buttons on - to do this, flip the key out and you'll see a small indent where the two halfs of the key meet.
Use a flat bladed screwdriver in the indent to separate the two halves, and you'll then see a code printed on the button-half of the key beginning with HLO then several digits. Mine was for a Golf MK4 - which was HLO 1J0 959 753 AG
Now what you need to do is buy a new/used key with the same code (just google it or search eBay). Once you have purchased the key, you will need an auto locksmith to extract your immobiliser and alarm codes from your car by plugging a device into the diagnostic port on the car. These codes are needed in order to program the new key to the car's alarm and immobiliser.
I managed to find a guy called Michael based near Bristol on 07833 783 317 (he gave me permission to print this) who extracted the key codes from my car's computer. Once you have the codes from your car, you should write them down in a safe secure place (NOT IN YOUR CAR).
Once the codes were extracted, Michael was then able to cut my new key, and to program the remote to both the alarm and immobiliser - but once you have the codes for your car, you can then use just about any auto locksmith (only a few of them are capable of extracting the codes) who can use the codes to program the new fob - you can also use VAG COM to do this yourself (around £10 on eBay). Then you just have to copy your key blade.
Michael did the whole lot for £60 in about 1/2 an hour. He's a really nice guy, so if you are buying a VW key on eBay, I can wholeheartedly reccomend him - a really nice guy, and just off the M5.
Now I have the codes safely stored, if I ever loose my spare key, I can use any local auto locksmith and give them my codes.
So my total bill was £100 for a brand new VW key, programmed, with the codes extracted for future use. This is half the cost of anywhere else I looked, and that wouldn't have provided me with the codes!
If you are thinking about buying a replacement spare key, give him a call and he'll make sure he can do it before you buy. He can also help with programming for other brands of car.
Hope you found this useful!