For me, the cost benefit is about entertainment. I recognize there have been studies that supposedly show that games can help develop or maintain certain skills, but for me it's more about learning the skill to experience the in-game reward. That's just for some games. For others, that element exists but the game is telling a story too. One that is punctuated by struggle, maybe battles, and the overcoming which leads to power ups and more story.
So the cost-benefit is that it costs time, but it pulls you out of end-stage capitalism and puts you in flow state, engaging in another world.
I would suspect, though, that if you're seeing video games through the lens of cost-benefit analysis, you might have trouble relaxing. People need rest.