R. L. Dabney has a sophisticated doctrine of the atonement. In fielding objections from Socinians and modern skeptics, he makes a number of important distinctions. Here he explains why the atonement is not quantifiable:
The Reformed divines are also accustomed to make a distinction between penal and moral satisfaction, on the one hand, and pecuniary payment, on the other. In a mere pecuniary debt, the claim is on the money owed, not on the person owing. The amount is numerically estimated. Hence, the surety, in making vicarious payment, must pay the exact number of coins due. And when he has done that, he has, ipso facto, satisfied the debt. His offer of such payment in full is a legal tender which leaves the creditor no discretion of assent or refusal. If he refuses, his claim is canceled for once and all. But the legal claim on us for obedience and penalty is personal. It regards not only the quid solvatur, but the quis solvat. The satisfaction of Christ is not idem facere; to do the identical thing required of the sinner, but satis facere; to do enough to be a just moral equivalent for what is due from the sinner. Hence, two consequences. Christ’s satisfaction cannot be forced on the divine Creditor as a legal tender; it does not free us ipso facto. And God, the Creditor, has an optional discretion to decline the proffer, if He chooses (before He is bound by His own covenant), or to accept it. Hence, the extent to which, and the terms on which Christ’s vicarious actions shall actually satisfy the law, depend simply on the stipulations made between Father and Son, in the covenant of redemption…
We do not view the atoning value of Christ’s sacrifice, as a quantity, to be divided out by pound’s weight, like some material commodity. We do not hold that there must be an arithmetical relation between the quantity of sacrifice, and the number and size of the sins to be satisfied for; nor do we admit that, had the sins of the whole body of elect believers been greater, the sufferings of the substitute must also have been increased; as when the merchant buys more pounds of the commodity, he must pay more money for his purchase. The compensation made to justice is not commercial but moral. A piece of money in the hand of a king is worth no more than in the hands of a servant; but the penal sufferings of a king are. One king captive would exchange for many captive soldiers. Hence, Christ paid, not the very total sufferings we owed, but like sufferings, not of infinite amount, but of infinite dignity.
Christ’s sufferings were vast; and the capacity for feeling and enduring conferred on His humanity by the united divinity enabled Him to bear, in one life-time, great wrath. Second. It is the great doctrine of the hypostatical union, according to Heb. ix:14, which grounds the infinite value of Christ’s sufferings. (See that doctrine, Lect. 39th.) As the infinite nature of the god, against whom sin is committed, makes it an infinite evil, although the act of finite creature, so the acts of Christ’s human nature in suffering, have infinite value, because of the dignity of His person.
~Lectures in Systematic Theology, (Zondervan, 1976) 503-504, 514-515