Undo your existing key and inside there will be a number, only buy a second hand one off ebay if this number is identical, you can also pick up a blank blade aswell, once this is all done you will need to get it cut at a locksmith who has a laser cutter, when this is done recode it to your car by following instructions from other threads on the site, Note some models have an imobiliser link inside the key and only audi can code it then.
hope this helps as ive just been through the same all in all should cost no more than £50 - depending on how much you win your key for i paid £25 for mine but it is like brand new :icon_thumright: