I do not think either one is more expensive than the other. Having two daughters and three sons you pay about the same no matter the gender.
With Girls you are spending a lot of money on clothes make up, hair salons, nail salons, and little money on entertainment.
With Boys you are spending a lot of money on sports equipment, toys, outdoor gear, and little money on clothes.
While this is not always the case it just proves that your going to be spending a lot of money on a child. Plus no matter what you are told they all eat you out of food within a day or two of going to the store.