I think materialism doesn't come into effect at a percent of income spent or a number of presents under the tree. Rather, it's a line you cross when you become psychologically more excited for, or dependent upon, receiving gifts than seeing family, religious celebrations, or giving to others.
I tend to get wrapped up in Christmas--probably carried away--and my sanity check is always my charity budget. It helps for me as a Catholic because the weeks leading up to Christmas (advent) is a minor penitential season, so there's a spirit of poverty to the preparations and festivities.
Probably the most grounding experience for our family so far this season was receiving a beautiful handmade Christmas card from a child we sponsor overseas. Out of all the gilt and trappings of our western experience, her simple message of love--shared between virtual strangers--helped put in perspective the meaning of Christmas.
