It's a tough question, but so is: what is just? Like justice value is a possibly unknowable, possibly immeasurable communal platonic. We forged, refined, and sanity check our justice system using a few simple tricks from measurement theory. We ensure that the commonly agreed on axioms are covered (i.e. murder is wrong). We try to reduce several "intuitively agreeable" outcomes into simple rulesets (i.e. possession is 9/10ths of the law). And so on... Yes it's hard and our current system is only an approximation of the communal ideal, but few would want to dismiss law altogether because such measurement systems can do a pretty good job.

Defining value by the workings of the market is simply a circular and technical argument. It's no more useful than defining justice by actual outcomes of court cases. For one thing, markets define value rather simply: a simple majority vote where present and future votes are discounted. It's a pretty good rule with the nice feature that it's distributed requiring no central planning, but it is not magic and does have some less than desirable corner cases such as, say, the state of everyone at the lower end of the bell curve in western countries for the last four or five years.