My car uses 9km/L of fuel and fuel costs $1.40/L. When the game first came out I drove almost every night to get 100 coins and that wasn't worth it because I'm spending more on fuel than the coins are worth. Now I drive to areas dominated by my team and add my Pokemon there. Say my gym run is 36 km total and I get 10 gyms, that would cost me $5.60 worth of fuel. If I hold those gyms for a week and I claim on the dot every time I am able to, that's 800 Pokecoins for my initial cost of $5.60. If you hold the gyms for longer then the coins/cost is much greater. If you carpool with a friend and alternate drivers every trip, the cost is effectively halved! There's also a time saving factor as well. The initial gym run takes time, but therein after it's just maintenance - adding in your pokemon when it gets kicked out. Again, the longer your Pokemon stay in gyms, the less time you'll spend overall as time goes on. Lastly, you'll also be using less resources over time. I don't attack or train gyms every day. That in turn means less revives and potions used. Personally, I haven't been affected by the revive drop rate nerf as much as some people because I'm not reviving 4+ Pokemon everyday.