Objection 1. It would seem that it is not always unlawful to give money for the sacraments. Baptism is the door of the sacraments, as we shall state in the III, 68, 6; III, 73, 3. But seemingly it is lawful in certain cases to give money for Baptism, for instance if a priest were unwilling to baptize a dying child without being paid. Therefore it is not always unlawful to buy or sell the sacraments.
Objection 2. Further, the greatest of the sacraments is the Eucharist, which is consecrated in the Mass. But some priests receive a prebend or money for singing masses. Much more therefore is it lawful to buy or sell the other sacraments.
Objection 3. Further, the sacrament of Penance is a necessary sacrament consisting chiefly in the absolution. But some persons demand money when absolving from excommunication. Therefore it is not always unlawful to buy or sell a sacrament.
Objection 4. Further, custom makes that which otherwise were sinful to be not sinful; thus Augustine says (Contra Faust. xxii, 47) that "it was no crime to have several wives, so long as it was the custom." Now it is the custom in some places to give something in the consecration of bishops, blessings of abbots, ordinations of the clergy, in exchange for the chrism, holy oil, and so forth. Therefore it would seem that it is not unlawful.
Objection 5. Further, it happens sometimes that someone maliciously hinders a person from obtaining a bishopric or some like dignity. But it is lawful for a man to make good his grievance. Therefore it is lawful, seemingly, in such a case to give money for a bishopric or a like ecclesiastical dignity.
Objection 6. Further, marriage is a sacrament. But sometimes money is given for marriage. Therefore it is lawful to sell a sacrament.
On the contrary, It is written (I, qu. i [Can. Qui per pecunias]): "Whosoever shall consecrate anyone for money, let him be cut off from the priesthood."
I answer that, The sacraments of the New Law are of all things most spiritual, inasmuch as they are the cause of spiritual grace, on which no price can be set, and which is essentially incompatible with a non-gratuitous giving. Now the sacraments are dispensed through the ministers of the Church, whom the people are bound to support, according to the words of the Apostle (1 Corinthians 9:13), "Know you not, that they who work in the holy place, eat the things that are of the holy place; and they that serve the altar, partake with the altar?"
Accordingly we must answer that to receive money for the spiritual grace of the sacraments, is the sin of simony, which cannot be excused by any custom whatever, since "custom does not prevail over natural or divine law" [Cap. Cum tanto, de Consuetud.; cf. I-II, 97, 3. Now by money we are to understand anything that has a pecuniary value, as the Philosopher states (Ethic. iv, 1). On the other hand, to receive anything for the support of those who administer the sacraments, in accordance with the statutes of the Church and approved customs, is not simony, nor is it a sin. For it is received not as a price of goods, but as a payment for their need. Hence a gloss of Augustine on 1 Timothy 5:17, "Let the priests that rule well," says: "They should look to the people for a supply to their need, but to the Lord for the reward of their ministry."
Reply to Objection 1. In a case of necessity anyone may baptize. And since nowise ought one to sin, if the priest be unwilling to baptize without being paid, one must act as though there were no priest available for the baptism. Hence the person who is in charge of the child can, in such a case, lawfully baptize it, or cause it to be baptized by anyone else. He could, however, lawfully buy the water from the priest, because it is merely a bodily element. But if it were an adult in danger of death that wished to be baptized, and the priest were unwilling to baptize him without being paid, he ought, if possible, to be baptized by someone else. And if he is unable to have recourse to another, he must by no means pay a price for Baptism, and should rather die without being baptized, because for him the baptism of desire would supply the lack of the sacrament.
Reply to Objection 2. The priest receives money, not as the price for consecrating the Eucharist, or for singing the Mass (for this would be simoniacal), but as payment for his livelihood, as stated above.
Reply to Objection 3. The money exacted of the person absolved is not the price of his absolution (for this would be simoniacal), but a punishment of a past crime for which he was excommunicated.
Reply to Objection 4. As stated above, "custom does not prevail over natural or divine law" whereby simony is forbidden. Wherefore the custom, if such there be, of demanding anything as the price of a spiritual thing, with the intention of buying or selling it, is manifestly simoniacal, especially when the demand is made of a person unwilling to pay. But if the demand be made in payment of a stipend recognized by custom it is not simoniacal, provided there be no intention of buying or selling, but only of doing what is customary, and especially if the demand be acceded to voluntarily. On all these cases, however, one must beware of anything having an appearance of simony or avarice, according to the saying of the Apostle (1 Thessalonians 5:22), "From all appearance of evil restrain yourselves."
Reply to Objection 5. It would be simoniacal to buy off the opposition of one's rivals, before acquiring the right to a bishopric or any dignity or prebend, by election, appointment or presentation, since this would be to use money as a means of obtaining a spiritual thing. But it is lawful to use money as a means of removing unjust opposition, after one has already acquired that right.
Reply to Objection 6. Some [Innocent IV on Cap. Cum in Ecclesia, de Simonia] say that it is lawful to give money for Matrimony because no grace is conferred thereby. But this is not altogether true, as we shall state in the Third Part of the work [Supplement, 42, 3]. Wherefore we must reply that Matrimony is not only a sacrament of the Church, but also an office of nature. Consequently it is lawful to give money for Matrimony considered as an office of nature, but unlawful if it be considered as a sacrament of the Church. Hence, according to the law [Cap. Cum in Ecclesia, de Simonia], it is forbidden to demand anything for the Nuptial Blessing.
Source: Summa Theologica (New Advent)