Pre-baby we were basically constantly in the 50-60% saving range. We had a baby in February. We have been knocked down to about 40% the last 6 months.
Its more the loss of my wife's income than the new costs of the baby, and she was a freelancer so no "mat leave" for us. The actual cost increase of the baby has been pretty small.
The big, big saver - have time for the baby! If you have time, you don't need to buy toys and all the high-sensory garbage that is 50% as effective as your face/voice/body. So I work from home, and I think its a big, big cost saver as well as mental saver for my wife so she's not stuck home alone all day.
The smaller things we did the following to save on money:
1. Borrowed ALL our baby clothes (or got them as gifts), probably $100+ per month of savings.
2. Borrowed lots of her mat clothes
3. Bought all used "staples"
4. Don't use wipes, clothes are a massive saving
5. Don't buy them stuff! You are enough. When you do (e.g. books), guilt trip grandparents, borrow or buy used
6. "Borrowed" receiving blankets, clothes and other supplies from the hospital. (I figure if I am paying hundreds of dollars, I was due!)
The Big Costs
1. The biggest cost is that we went from no car for many years, to finally owning a car. Still torn about this one. You can't bike with an infant < 6 months, and bus's in the area are "hit and miss" for being able to get a stroller on them easily. We spent two hours chatting tonight about if we're ready to give up the car or not. Really tough call since I work from home and we ONLY need the car for personal reasons. I think in the next 6 months we'll ditch the car, I think... hard to say... with insurance, gas, etc this adds up to $400/month. Yuck.
2. Insurance, we had to get life insurance & critical illness for me since I am now the only bread winner and had dependents. The life will stay on until kids are 18+, but the critical illness we'll drop after 1 year when my wife is able to work again. (I am technically self-employed, so I have to self-insure). About $75/month.
3. Last but not least, diapers! Haven't found a solution to this yet, but basically $15 x 4 weeks = $60/month. I just don't buy that cloth diapers are cheaper, they end up getting stained & replaced.
There have been some unexpected cost savings in the lifestyle changes:
1. alcohol - hangovers + babies = no fun. When we drink its now only a drink or two... and we never have booze in the house unless there are guests
2. The occassional restaurant treat is a thing of the past.
3. Almost all of our free time is spent walking in parks with the baby, you can't really do "cultured" activities (museums, movies, music, etc) with an infant. All you can really do is walk and talk. It has actually been fantastic for our marriage!