Being "Cost effective" is a term that means a player who has invested a game resource or currency into something has gotten equal to or more currency and/or strategic success out of the investment. Another way of saying this is simply that a purchased item or unit justified its cost, or "payed for itself". This is primarily a term used in RTS games that has some applications in Starhawk. Being cost effective can be important for slowly pulling ahead and staying there for a game.
A possbile explanation of how being cost effective applies in Starhawk can be used with structures. All structures cost rift energy and can cost more as the game goes on. More specifically with Hawk Hangars. They cost (6) blocks of Rift Energy to drop, which is a noticably higher cost compared to other structures. Getting enough Rift for a Hawk Hangar, and using the Hawk yields enough kills to buy another Hawk (2), that can be considered a cost effective use. If the Hawk does significant damage to other enemy Structures, or grants other strategic successes like map control, it can also be considered cost effective. However, if the hawk that was spawned is killed off quickly without doing much damage, that can be considered cost in-effective, and a waste of recources that must be regained. Moreover, if an entire hawk hangar is destroyed, the team must invest another 6 rift energy into a hangar and can fall behind in the hawk count and slowly lose air control and eventually map control in general. The enemy team on the other hand, still has their structures up and can invest into more buildings or bank their rift energy for later in the game for Upgrades or replacing structures. This one of many possbile instances that involve being cost effective.