Great advice. Another way to think of it is to look for areas that companies often have teams of people working for weeks (or months) to solve a problem that you can solve in a fraction of the time. If they are spending $100K to not solve the problem, asking for $120K to solve it in two months is a good deal for both sides.

You can never get more than your perceived differential value, but buyers will be happy to pay you far less if that's OK with you. ;-)