How much does it cost to be a stay-at-home mom?

In 2009, almost a million more American families with a child under the age of 6 included a dad who worked and a mom who stayed at home than in 1986. That's a big number, but percentage-wise the rate of married couples with children that include a stay-at-home mom actually declined between 1986 and 2009, from 44 to 37 percent.

The advent of power moms, those women who pour as much drive into their families as childless women put into their careers, have made stay-at-home moms much more visible. Power moms are celebrated in advertising. They've also shown they're capable of launching social media campaigns against companies that miss the mark. Power moms are fashioning themselves as the new standard for motherhood. The reality is, however, that more mothers are opting to maintain both careers and families than are opting to stay at home.

Certainly, one reason for these figures is the abundance of choices afforded to modern women. Working mothers benefit more from flexible schedules and hours in the workplace, to be sure. But another reason that more families don't include a stay-at-home mother is because it's simply too expensive. A 2009 survey conducted by the human resources firm Adecco found that 80 percent of working mothers said they worked because they had to.

The result of giving up income in favor of raising kids is what has come to be known as the "mommy tax". The most prominent figure bandied about for the total loss of income that constitutes this so-called tax is $1 million. But what are the factors that add into this mind-boggling number, and is it really that big?

Calculating the Mommy Tax

Personal investment guru Ric Warren famously calculated that the services provided by the average housewife -- the cleaning, the errands, the chauffeuring and myriad others -- are worth an annual salary of about $500,000. The problem is economic realities don't hold that much value in the work provided by stay-at-home moms and housewives. This means that when a mom decides that she's going to stay home with her child, she's opting out of the chance to generate cash for the household.

The most obvious factor in the so-called mommy tax is loss of a second source of income. If a mom makes $50,000, that income dries up when she opts to stay at home. The amount lost is less than that round figure, however. Federal and state governments remove tax contributions from income, and the feds also require contributions to Social Security and Medicare. This makes the loss of income less than it initially appears.

If mom's income is no longer in the picture, the couple must still file jointly. This reduces the taxable income, which reduces the household's tax. Taking into account federal taxes, the real loss of income to the family is $39,300, not the $50,000 in gross income. And this is just factoring in federal income tax alone, not other required contributions. To get a real idea of what annual income loss the family will see, multiply the net income on your pay stub by 12 months.

A little more nerve-wracking is the loss of any savings investments contributed by the mom's earnings. The thought of cutting a potential nest egg in half is daunting. This can be overcome, especially in cases where the mother's employer matched contributions to a company 401(k) or IRA. This definitely represents money lost that can't be regained in other areas.

Health care benefits are also a consideration. In many cases, the dad's employer offers enough coverage that the entire family will be covered. A family may already lean on just one parent's plan. If this is the case, it's likely the better, less expensive plan offered between the two employers. If mom's plan proved better and she opts to stay home, then look for an increase in monthly insurance premium payments deducted from dad's paycheck. To be clear, even though you can make it with just one plan, the loss of health benefits still equates to a loss of income.

Staying at home doesn't represent a total loss, however.

Gains from Staying at Home

There is one very big expense that career moms have that stay-at-home moms don't: daycare. On average, Americans pay about $679 a month, or $8,148 per year, on child care expenses. That's a pretty big chunk of livelihood, considering the average monthly expenditure on housing in 2009 was $1,425.

If we're taking a hard look at exactly how much income an American family loses when the mother opts to stay home, we must factor this in, too. Remember, we arrived at $39,300 lost on the last page? Subtract another $8,148 previously spent on daycare and we're at $31,152. Another way to look at it is that the hit your family's monthly income took at its worst level just got bigger -- by $679 per month.

Beyond the numbers lies a different value that's harder to factor in. Economists have long struggled to quantify esoteric, but very real, factors like personal and familial well-being. These play a role in deciding the real cost of staying at home to raise children. Remember that survey that found 80 percent of working mothers said they work because they have to? That same survey also found that 48 percent of working moms report wanting more time to spend with their kids. For some working moms, there's a certain amount of guilt associated with juggling a career with a family.

There is also a long-standing struggle over studies delivering mixed results when examining how kids fare with moms at work or home. For example, a recent British study in the Journal of Epidemiology and Community Health found that kids whose moms stayed at home ate healthier and were more likely to play in organized sports. While these studies tend to raise a larger debate over women's role in society, not everyone has the option of staying at home. Just as the amount of actual income lost is different for each mom who opts to stay at home, the factors involved in choosing to work or not work vary as well. For a mom who longs to spend more time with her kids, it may be worthwhile to take the time to crunch the numbers on exactly how much staying at home will cost, and if her family can afford it. The answer may be pleasantly surprising.