I study kinetic metabolic models under an "economical" aspect, i.e., assuming an optimisation of enzyme levels. To characterise optimal metabolic states, I introduce economic potentials, dual variables that quantify the "usefulness" of individual metabolites. In optimal states, the potential differences, multiplied by the flux, must be balanced by positive enzyme costs; accordingly, the potentials tend to increase along pathways. This postulate can be employed as a constraint in flux modelling, in order to exclude futile flux cycles and other flux patterns that are incompatible with an optimal allocation of enzyme. The new economic constraints on fluxes resemble well-known thermodynamic constraints, and methods from thermodynamic flux analysis can be reused to characterise optimal metabolic states.