Well as far as the kids' clothes go, it really kinda evens out. I usually buy inexpensive clothing for the kids anyway as they grow out of them so darn quickly. I find that it's less painful to sell/give away those clothes when I didn't pay that much for them to begin with.
For my clothing, it really is cheaper. I can get fabric for myself because a lot of stores have more "adult" type fabric on discount that they do childrens' fabric. I have hit a lot of good bargains in the dollar bin at wal mart and have made myself a number of shirts for about 2$ (includes notions, such as elastic, thread, bias tape, etc)
I started out sewing because I wanted to save money. Now? Nope, I spend more on fabric and notions than I EVER would have on clothing. Not to mention the 4 sewing machines and the serger, countless yards of fabric in storage, special elastic, snap setter, etc.
So you CAN do it, you can make your clothing and the cost CAN be considerably less than store bought items but it usually ends up becoming an addiction and you'll probably end up spending more money on fabric than you can possibly imagine. Now, I'm not saying that to dissuade you at all. I'm just giving you a little forewarning
First a sewing machine, then a serger, then a better sewing machine, etc...If you can control yourself, great! But me? Nerp, I love fabric and notions!