The law of supply and demand is the bedrock principle of economics. Most often the idea is expressed as a simple function of price: When price decreases, supply decreases while demand increases. When price increases, supply increases while demand decreases. Common experience confirms what economists teach. Every worker wants a job that pays more, for example, so that supply of labor increases as wages increase; just as every consumer wants to pay less for tennis shoes, so that consumers buy more tennis shoes marked down than at regular price. One consequence of the law of supply and demand is that of shortage and surplus: When the price of a good exceeds the market-clearing price, a surplus results, and when the price is below the market-clearing price, a shortage results. Everyone wants the most bang for the buck.
The truth of all this holds not just for tangible goods, but for most anything imaginable that could potentially enhance human well-being. As economist Roger Arnold has noted,
[A] good is anything from which individuals receive utility or satisfaction. In everyday conversations, the word good usually applies to something tangible that is bought or sold in a market. But there are more goods in the world than just the tangible items sold in markets. Friendship and love are both goods, although neither is tangible and neither is bought and sold in a market...
What about the Christian life, or as some have called it, "the good life"? Clearly a majority of people, in America at least, consider being a Christian somewhat valuable. With various levels of zeal we support Christian causes, read Christian books, attend Christian churches, defend Christian causes. Professing Christians are everywhere you look. At the same time, skeptics and critics point out that in behavioral terms Christians are scarcely distinguishable from anyone else: Indeed, statistically we are no less likely than anyone else to divorce, have children out of wedlock, get caught in a financial scandal, or commit a violent crime.
Why the inconsistency? Perhaps economics can provide further insight. If the church is experiencing non-stop numerical growth with little spiritual growth to show for it, the problem may have to do with what Dietrich Bonhoeffer referred to contemptuously as "cheap grace." That is, the advertised price for following Christ is simply too low and consequently everyone wants in on the deal. But the Christian life is not cheap. It cost Jesus an agonizing death to provide us access to eternal life and communion with himself, the same Jesus who directed that each of his disciples would have to "take up his cross" in order to follow him. It should not surprise us, then, that Jesus compared the life of discipleship to a costly all-out war or an expensive long-term building project, and then urged us to "count the cost" before presuming to be his disciples: "So likewise, whoever of you does not forsake all that he has cannot be My disciple" (Luke 14:33).
On this point I believe the skepticism of our atheist friends may prove illustrative. Atheists are quite unwilling, after all, to acknowledge Jesus as Savior and Lord, let alone as God incarnate. Now most often this aversion to faith is passed off as a strictly intellectual matter – Christian theism is incoherent, there is no evidence for it, and so forth – so that atheism is said to be more rational than Christianity. But the argument could be made that atheism is not so much intellectually but rather economically rational. That is, if genuine faith costs a person everything, atheism (often defined as simple "lack of belief") would be the rational outcome of simply refusing to pay such a high price for faith. From that perspective casual Christianity is no more, or less, rational than atheism. Supposing that there is no God makes as much sense as supposing that Christ places no demands upon us.
 Roger Arnold, Microeconomics, South-Western, 2001. At the moment I don't have access to my old college text from which I originally took this quotation, so I don't know the page number. But Arnold has made similar statements elsewhere.