Boro offered £1million for the Icelandic international - less than half AZ's asking price
Angry Van Gaal said: "Their offer is nothing - £1m for a player like that amounts to a waiter's tip."
Van Gaal also said he did not expect Boro to play as well as League One Hartlepool.
AZ drew 1-1 at Victoria Park in a midweek friendly.
Van Gaal said: "I don't know if Middlesbrough will be much better than Hartlepool."