I wouldn't pay too much of a premium for the Anniversary over a standard GTI. They should be around the same price so I reckon 4.5-5k is about right.
If you do find one, make sure you know what extra bits it should come with. I saw a few advertised which claimed to be anniversary's but were missing some of the extra trim.
They are more common than the sellers think, so don't let them give you the sales spiel about special edition etc.