I can't speak for sewing, but as a knitter, I know that knitting your own things is not usually cheaper than buying ready made.
It depends on whether you compare like for like, though. Obviously a hand knitted cashmere jumper is going to be a lot more expensive than an acrylic one from Primark. Most knitters I know wouldn't bother knitting something out of cheap yarn, as they know they could get something of similar quality for the same money and no effort.
Comparing a hand-knit with a high-end garment is very different, as you can pay serious money for a well made jumper made of expensive yarn, and in that case, making your own is cheaper, and you can be sure that the fit is right.
I factor in that knitting is my hobby, so see the expense in terms of what I would have spent on a different pastime, and tell myself that I am also producing clothes that I would have otherwise had to buy (if I could afford to pay for cashmere or silk clothes, that is).