I'm about to have a baby in a few weeks, and it would cost us more if I didn't work and stayed home. The cost of daycare is significant, but it isn't nearly what I make after taxes, and I got a 10% raise last year with a promotion. The daycare we'll use is in my building, what's more, so I won't be far away.
Gas/car wear and tear--I would actually drive my car
more if I stayed at home. DH and I carpool to work. I wouldn't want to stay in my house all day every day. I would visit friends/family, go to the zoo, and do the grocery shopping, etc. while DH is at work, instead of the both of us running errands together on the way home.
Clothes--wouldn't make much difference in cost. I would wear more jeans instead of slacks. I would probably wear less makeup, but I wear so little it wouldn't make much difference.
Our health care is through my job, it would cost more through DH's. We're both contributing to retirement accounts. It's VERY important to me that we have enough money for retirement and can afford health care in our old age. DH is a type 1 diabetic, I don't know how long he'll be healthy. I cannot count on him being able to work forever, what's more. If we divorced and he died while he was still working, I couldn't get any survivor benefits for myself, only for our minor child.
Also, I plan on going back to school to get a master's degree. My employer will help pay for part of it.
In many cases, I do think that one spouse quitting to stay at home with the kid(s) can be beneficial to the budget, not detrimental. But this depends on many factors. Most particularly--daycare costs for multiple children. The more you have, the more $$ you spend. If we had 3 kids, it would be better if one of us stayed home. But with one kid (who will be the only child DH and I plan on spawning), the daycare costs won't make much of a difference.
Besides the above, eventually, our kid will go to school, most likely public. Childcare costs will plummet. At that point, I would feel ridiculous if I stayed at home all day. I would also have missed out on years of increases of my current salary, possible promotions, tuition benefits if I still chose to get a master's degree, retirement benefits would be lessened, I might be less marketable in the professional world, and so on.