I'm lucky in that I am splitting the difference between being a SAHM and a working mom. I work for my company part time from home. I figured it out and I am am bringing home about the same as I was when I commuting and working twice the hours. I commuted about 50 miles round trip so I was filling up my car 1-2 times a week, I was on the road a lot for meetings (got mileage for that) so I ate out a lot, work clothes, cleaning person, daycare, dry cleaning, etc. I swear I was getting my oil changed every other month and putting tires on my car all.the.time. We bought my current car brand new about a year and a half ago and I just had my third oil change earlier this week! I would also spend more money because there were days I needed to get out of the office so that usually meant a trip to Target. I also had a lot of mom guilt that I tended to make up with "things" because I just didn't have the time in a typical day with my daughter.
Now I eat at home, fill up my car with gas maybe every 2-3 weeks, clean my own house, iron clothes, able to coupon more, do free activities during the day (because face it, there are tons of free stuff to do with kids during the day, not so much after work), wear more casual clothes, don't have daycare expenses, able to get on my DH's insurance without penalty for less than what I was paying (and it is better so less out of pocket medical costs), etc. I still get 401k match and eligible for the annual bonus.
The biggest "savings" though is just my time. I no longer spend almost two hours commuting a day in addition to working a full day. I wake up and can be logged in and working within 10 minutes. I can work around my schedule so if that means before my kid gets up or when she is in bed, that means it is super easy to make my hours and get my work done AND still have time to essentially be a SAHM. It isn't unusual for me to be "done" with work before my daughter wakes up for the day. It's nice that everything like errands and shopping is done during the day so nights and weekends are family time doing whatever.