▁the ▁imper fe ctions ▁of ▁the ▁price - form ▁itself , ▁which , ▁the ▁fet ish ization ▁of ▁price ▁statistics ▁not with standing , ▁permitted ▁the ▁expression ▁of ▁product - values ▁only ▁in ▁an ▁approximate ▁way ▁( see ▁real ▁prices ▁and ▁ideal ▁prices ). ▁At ▁best — Mar x ▁assumed — there ▁was ▁a ▁reason ably ▁close ▁correspond ence ▁between ▁total ▁product - value ▁and ▁total ▁production ▁prices . ▁He ▁believed ▁economic ▁fl uct u ations ▁imp lied ▁that ▁if ▁some ▁products ▁were ▁sold ▁below ▁their ▁value , ▁this ▁necessarily ▁meant ▁that ▁other ▁products ▁were ▁sold ▁above ▁their ▁value , ▁and ▁vice ▁vers a . ▁The ▁diver gence ▁between ▁total ▁product - values ▁and ▁total ▁production ▁prices ▁on ▁the ▁whole ▁was , ▁Marx ▁believed , ▁probably ▁not ▁so ▁very ▁large , ▁in ▁an ▁open , ▁compet itive ▁market ▁within ▁the ▁domestic ▁economy , ▁where ▁enorm ous ▁price - value ▁dis cre pan cies ▁were ▁ordin arily ▁impossible ▁to ▁maintain ▁commer cially ▁for ▁any ▁length ▁of ▁time . ▁ ▁A ▁meas uring ▁unit ▁of ▁value ▁can ▁exist ▁only ▁as ▁a ▁theoretical ▁entity ▁( or ▁as ▁an ▁ideal ▁price ▁compar able ▁to ▁an ▁empir ical ▁price ) ▁which ▁is ▁also ▁exactly ▁how ▁Marx ▁used ▁it ▁in ▁his ▁simplified ▁illustr ations ▁of ▁value ▁relationships . ▁He ▁simply ▁uses ▁a ▁number ▁for ▁the ▁value - quantity ▁and ▁another ▁number ▁for ▁the ▁price - quantity , ▁to ▁indicate ▁a ▁proportion . ▁Emp ir ically , ▁one ▁can ▁only ▁get ▁as ▁far ▁as ▁establish ing ▁a ▁" grand ▁average