The Philosopher notes in the justice in exchange that value is a measure of demand. It is that we want or need something that we engage in an exchange in the first place. If there was no demand, then there would in fact be no economics. Human need then is a principle economics and the seeking after things that are external to us.
However, demand remains on the subjective side. It is from our own desires and ends that we determine our needs. The value of a thing is then in its usefulness in fulfilling those desires and meeting those ends. As the Angelic Doctor teaches, the value of a thing is not based on its intrinsic nobility, otherwise a slave would be of greater value than a horse, but rather its usefulness to man.
A thing may be useful more or less in itself. It is grounded in reality that we find whether or not something can actually meet the ends that we seek. In this sense, economic value is firmly grounded in reality. It is fundamentally extra-mental. However, the natural powers of a thing as intrinsic to it are not sufficient to a complete understanding of economic value, for there are many things will many powers to greater and lesser degrees that are of no economic value. Moreover, we seek that some things which have intrinsic usefulness where at one time considered worthless, but now are of great value. Uranium has the power to produce great energy when properly arranged with other materials and is thus highly valued, but this was only so after the 20th century where such energy was highly sought and methods to obtain it had been developed.
It is then apparent that economic value finds its grounding in the real intrinsic powers that a thing possesses. However, value formally and actually does not arise in the extra-mental but in the mental, formally as it is a measure of desire which is mental and actually as we have seen from above that if it is not useful to man it is not actually valuable.