DD2 didn't cost too much to begin with. Having two the same gender saved us a fortune in clothing (the initial huge costs were my fault though given my inability to stop buying cute baby stuff!). Small expenses in topping up the nappy stash etc but otherwise pretty good.

When it did get expensive is around the 2.5 yr old stage. Two kids doing swimming lessons, or gym classes, or dancing classes or whatever is a big price increase. They stop being able to share/hand down clothes so that increases. Now that both are at private school (again, our choice so its not necessarily an expense you HAVE to have) there's another big chunk out of our budget.

We had the same thoughts you're having when we were discussing the pros and cons of a #3. We probably can't afford it but we'll make it work.