Objection 1. It seems that magnificence is not a part of fortitude. For magnificence agrees in matter with liberality, as stated above (Article 3). But liberality is a part, not of fortitude, but of justice. Therefore magnificence is not a part of fortitude.
Objection 2. Further, fortitude is about fear and darings. But magnificence seems to have nothing to do with fear, but only with expenditure, which is a kind of action. Therefore magnificence seems to pertain to justice, which is about actions, rather than to fortitude.
Objection 3. Further, the Philosopher says (Ethic. iv, 2) that "the magnificent man is like the man of science." Now science has more in common with prudence than with fortitude. Therefore magnificence should not be reckoned a part of fortitude.
On the contrary, Tully (De Invent. Rhet. ii) and Macrobius (De Somn. Scip. i) and Andronicus reckon magnificence to be a part of fortitude.
I answer that, Magnificence, in so far as it is a special virtue, cannot be reckoned a subjective part of fortitude, since it does not agree with this virtue in the point of matter: but it is reckoned a part thereof, as being annexed to it as secondary to principal virtue.
In order for a virtue to be annexed to a principal virtue, two things are necessary, as stated above (Article 80). The one is that the secondary virtue agree with the principal, and the other is that in some respect it be exceeded thereby. Now magnificence agrees with fortitude in the point that as fortitude tends to something arduous and difficult, so also does magnificence: wherefore seemingly it is seated, like fortitude, in the irascible. Yet magnificence falls short of fortitude, in that the arduous thing to which fortitude tends derives its difficulty from a danger that threatens the person, whereas the arduous thing to which magnificence tends, derives its difficulty from the dispossession of one's property, which is of much less account than danger to one's person. Wherefore magnificence is accounted a part of fortitude.
Reply to Objection 1. Justice regards operations in themselves, as viewed under the aspect of something due: but liberality and magnificence regard sumptuary operations as related to the passions of the soul, albeit in different ways. For liberality regards expenditure in reference to the love and desire of money, which are passions of the concupiscible faculty, and do not hinder the liberal man from giving and spending: so that this virtue is in the concupiscible. On the other hand, magnificence regards expenditure in reference to hope, by attaining to the difficulty, not simply, as magnanimity does, but in a determinate matter, namely expenditure: wherefore magnificence, like magnanimity, is apparently in the irascible part.
Reply to Objection 2. Although magnificence does not agree with fortitude in matter, it agrees with it as the condition of its matter: since it tends to something difficult in the matter of expenditure, even as fortitude tends to something difficult in the matter of fear.
Reply to Objection 3. Magnificence directs the use of art to something great, as stated above and in the preceding Article. Now art is in the reason. Wherefore it belongs to the magnificent man to use his reason by observing proportion of expenditure to the work he has in hand. This is especially necessary on account of the greatness of both those things, since if he did not take careful thought, he would incur the risk of a great loss.
Source: Summa Theologica (New Advent)