How expensive depends on how you teach your children. Teaching them to use what they have wisely (money, clothes, toys) early on prevents them from being a big financial drain later on. For starters, don't buy them too many toys. The teenage years get real expensive (from middle-school on) since he'll probably need new clothes every year due to growth. That, plus a growing appetite. Oh, and then college is a big hit (needless to say).

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2014 Mazda 3 GT: Ø Consumption: (click for details)