a better solutions is simply make the formula fit the situation

like based on the food u have in the city ...

big players with stock piles will cost them smaller players with small amounts or any player with a city that has been 0'd will cost less ... set a min and a max but by baseing it on food in the city you are leveling the playing field giving a person a chance to defend via comfort which is a defense. sometimes ur only defense if u were attacked and 0'd when u were off line ... shouldnt be punished for each use that just rewards the attacker ... It has always been hard or imposible to take a city off a person who is online now ur saying that doesnt even matter. So stack the cards in the attackers hands and on the side of botters ... GREAT JOB EVONY great job