I don’t recommend giving money because it could be easily mistaken as a Mass stipend. Worse, it could also be mistaken for the kind of “end-of-year gratuity” you’d give your newspaper delivery person or hair stylist, and that would be offensive.
Why not give your priest a Christmas card, including in it a letter of appreciation? If you know his tastes, you could also give him an appropriate gift such as a book, a CD, a DVD, or a luxury food (e.g., specialty coffee, sweets); or, if you don’t know him well, you could give him a gift certificate (e.g., restaurant, movie theater, bookstore).
