See the OVN wiki for some of my ideas on value.
As part of the Sensorica network, playing a lead role in designing the OVN model, my working definition of value that I proposed is:
Value rests on a subjective experience that moves human beings into action.
Think of value as a triadic relation between a valuable (a reality, a desired object), a token (can be money, but also a symbol that doesn't represent a monetary currency) that stands for it, and a valuation, which is in part subjective and in part inter-subjective, i.e. shared, through culture and socialization.
This follows the same pattern as Peirce's theory of semiotics, the triangle of reference or semiotic triangle. In this case, the token is a representation of a desire/need that can be matched with a representation of anther desire/need. This definition can model non-monetary transactions like barter, as well as exchanges mediated by a monetary currency, like a purchase.
Note that this definition can be extended to other living systems, it is not only reserved to humans.
Valuation of something is an imaginary process by which an individual projects him/herself into the future trying to imagine how moved he or she would be by that same thing, or into the past, trying to remember an experience where he/she as moved by the same thing, modulated by present predispositions towards the same thing. The result of this process is circumstantial and characteristic to that specific individual.
more on the OVN wiki.