Complex sentence: Whether the net effect on the trade balance is positive or negative depends on whether or not the quantity effect outweighs the cost effect; if the quantity effect is greater, then we say that the Marshall-Lerner condition is met.
Split into simpler sentences: Whether the net effect on the trade balance is positive or negative depends on whether or not the quantity effect outweighs the cost effect. If the quantity effect is greater, then we say that the Marshall-Lerner condition is met.