Felix said:The result is that less powerful abilities like Silent Moves increase the armor's price geometrically as you add more on, while more powerful abilities like Spell Resistance increase the price exponentially.
Actually, I think you mean that less powerful abilities increase the price arithmetically (or linear, an adding rule), while more powerful abilities increase the price as a square (I guess you could loosely use either geometric or exponentially for this latter item, but it's not technically either).