Value actually is an adjective, but it might not sound entirely natural in this use. While it's used in such phrases as a value proposition, that's not the case here, and a "value axiom" might not have exactly the sense you are looking for.
If you are looking for an adjective that doesn't just mean that something is about value, or will deliver value, but how it's valuable, how to calculate its value, or what the "source of value" is, then consider valuational.
From its noun form of valuation:
1 : the act or process of valuing
specifically : appraisal of property
2 : the estimated or determined market value of a thing
3 : judgment or appreciation of worth or character
Its adjectival form would produce the following:
A valuational axiom.
Some examples of its use include the following. I have emphasized all instances of the word.
From Institutional Economics: Theory, Method, Policy, edited by Marc R. Tool:
Tool sees the act of analysis as valuational. Moreover, he sees valuational criteria as provisional and subject to the same process of valuation as as the actual analysis of the cultural processes.
From Free Will: Critical Concepts in Philosophy, edited by John Martin Fischer:
The possibility of unfree action consists in the fact that an agent's valuational system and motivational system may not completely coincide.
From Fact and Value in Emotion, edited by Louis C. Charland and Peter Zacher:
Alexithymia can shed light on two main interconnecting themes of this volume. First, the designation of psychopathology includes not only a value judgment, but in fact a judgment that there is something wrong with the valuational aspect of a person's functioning. A clinical diagnosis is in part a valuational judgment of another person's capacity for making valuational judgments.