Unfortunately, there is no real way to discover equivalence since different vocations deal damage in slightly different ways.
A mage will have a more consistent damage output due to the fact that their damage spells never "miss" except in the case of player error.
A knight has a wide range of damage and can hit for as low as 0 with a melee attack, or can deal a critical hit to every monster around him dealing significant damage. A knight is limited in his damage due to the fact he can only hit at most the 8 squares around him with most of his spells. A knight can get high combos a single target utilizing combination of auto attack and spells, something fairly unique to this class as he can do it at any time.
A paladin can deal extremely high single target damage, or he can deal high area damage. A paladin is a strong inbetween of a knight and mage where they can deal consistent damage to area targets, or high number single target damage that faces the same possibility of no damage as a knight.