*per- (5) Look up *per- (5) at Dictionary.com
Proto-Indo-European root meaning "to traffic in, to sell," an extended sense from root *per- (1) "forward, through" via the notion of "to hand over" or "distribute."

It forms all or part of: appraise; appreciate; depreciate; interpret; praise; precious; price; pornography.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit aprata "without recompense, gratuitously;" Greek porne "prostitute," originally "bought, purchased," pernanai "to sell;" Latin pretium "reward, prize, value, worth;" Lithuanian perku "I buy."