I Have a small List of around 70 people in which around 25 are people who bought previously from me. Whenever I sell a small 7$ report to my list i get around 5-10 sales making me around 35-70$ every time.
This proves that how important list building is for a marketer. Whenever I have to pay for my hosting i promote a product to my list which makes me enough to cover my hosting fees.
Just imagine how much you can make if you had a list of 1000's or even bigger.