Please don't wear a crewneck under a button-down, that just looks awful. Also I can't imagine I'd ever want to wear a long sleeve tee under a dress shirt either. Finally, $30 for a Gap tee = If it's so cold that you really need to wear something under a button down, get some v-necks with the v deep enough that it isn't visible. Hanes or Jockey or whatever in the 3 pack will do the job. Or some wifebeaters.