To take it a little further, if all else were equal, you could estimate the cost of an open jaw as the sum of half the round-trip tickets to each city, so something between the two costs. But things are not always (maybe rarely are) equal. Both cities need to be well served by the same airline or alliance to get the best price. The existance of specials or discounts will distort prices as well. Keep in mind the reason for doing open jaw is to eliminate the need to backtrack, adding travel cost on the ground. If your itinerary does not benefit from this, then round trip is probably best.