For whatever reason, Kabam decided that a few champs should cost more to level up than others. Unstoppable Colossus and Ultron also take increased iso to level up. To my knowledge, Kabam has never specified a reason for this distinction.
Premium champs like @Mirage_Turtle mentioned take more iso than normal. I think also fully robotic ones like sentinel, ultron require more. I think immortal iron fist is included there too. Don't know the reason for it tho since it's never been stated by Kabam.
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For whatever reason, Kabam decided that a few champs should cost more to level up than others. Unstoppable Colossus and Ultron also take increased iso to level up. To my knowledge, Kabam has never specified a reason for this distinction.