I just watched Film theory's video on Fullmetal Alchemist's value of a human soul each one is worth around 654 terrajoules.
As a crossover, per BoVD's Souls as Power each generic unspecified soul is worth of 10xp when used for exchange giving us a 11xp=65tj figure. Since Magical Items consume one twentieth-fifth of their Market Cost in experience points, for every 1,000gp you need to invest around two petajoules worth of energy.