I do that all the time. In fact, most of the time when I'm out shopping I have a mini notebook with me to sketch all the things I like and could probably make myself.
I remember going out with a friend to see Toy Story 3 and we had some time to kill. So we to this store with real expensive clothes to window shop. I found this top that was in the clearence rack. It was basically a flimsy grey t-shirt with the hems cut off, cut down the middle, and the middle was replaced with a black zipper. They wanted like $20 for it...this was off the clearence rack! I told my friend "$20 for THIS? I could easily make this for under $10 AND in colors I actually like!"