A friend asked for more clarification on what is meant by cheap law. I suggest that clarity on this point needs to be shocking. Cheap is the label for something that is inferior in quality. Sometimes that is seen as a good thing if you are looking for a deal at a yard sale. But in the world of personal relationships cheapness conveys insult and violation of what is good. Call a friend, a gift, or a woman cheap and see what reaction you get. This label is insulting, rude, or even crude. God is good and will only relate to his creation in a good way. This means that he isn't stingy, he won't sell himself, he won't lower the standards of goodness, and the surprise is that he is generous. God and the human conscience both testify that perfect love is always required and that evaluation by the standard of perfection is good.
To think or say that God doesn't require perfection, or to say that partial obedience is somehow acceptable to him, is to cheapen (or make inferior) his goodness. This is human pride attempting to mock God in order to get some credit (glory), but God will not be mocked. The person who does this is actually mocking himself and lying about God.
Why was Old Testament Israel often called a harlot—even said to be chasing and paying the men? And why after they claimed to have cleaned up their act did Jesus still call them adulterous? It was because Israel taught and acted like God sold his love in exchange for their payment of cheap imperfect obedience. They even had the gall to declare that they were obedient and God's judgment was really his inability to take care of them.
God is good; his law is perfect and good; his favor is good. But it would be prostitution for him to cheapen his standards and sell his favor for a little polluted partial obedience. To think that he does this is to do it oneself. Cheapness in relationships is prostitution. Human pride loves prostitution, but God hates it with a passion. God gladly offers his favor to all, but that favor must be received in a good way: either by perfect obedience to law or perfect freedom from law—by rest in the finished work of Christ.