My being a stay-at-home mom means that our family does not pay for daycare, extra transportation, or pre-prepared or restaurant meals. I also don't have to buy a professional wardrobe (including the cost of dry cleaning), or pay any other work-related expenses. When you factor in how much each of these things cost, that's huge! I have considered this before, but never looked at it quite in this way.
Generally the feeling is that a second income is needed just to meet the family's basic expenses in today's economy. But the reality is, once you do the math, many second income workers bring home less than 1/3 of their income, since they also have to pay for the above expenses that a family with a SAHM doesn't.
Of course, this is a very personal decision and not one to be made lightly, and not every woman can be a SAHM. (If you're trying to weigh the pros and cons, see this great article.) But, as my friend pointed out, just by staying home and taking care of my family's needs from home, I am saving my family a huge amount of money. I appreciate having that perspective. And, of course, the emotional and spiritual investment I am making in my family's well-being is even greater.