We haven't done anything differently either. We earn a certain amount, so we spend a little bit less than that. That's as complicated as our budgeting gets.
Babies can be dirt cheap or incredibly expensive depending on what you want. It's enirely possible to be comfortable on a low income, or struggle on a high income, you know?
As they get older they tend to cost more as you have all the extra 'activities' - some like swimming are almost a necessity, others less so, but it depends on you - and then school fees if you want to go private and so on. But this is all you can plan for years in advance.

First time round I had to go back to work very early on as DH did not have a stable income. This time he is working so I don't have to for a while. Much better.