If you describe the cost of someone's work as cheap, you disapprove of the way people are taking advantage of a situation to pay someone less than they should for the work that they do.
If you describe someone's remarks or actions as cheap, you mean that they are unkindly or insincerely using a situation to benefit themselves or to harm someone else.
If someone does or buys something on the cheap, they spend less money than they should because they are more concerned with what it costs than with its quality.