I think the best thing you could do, if you want to make money and not alienate the entire community is allow people to purchase enough ISO to level a character to level 141 for about $10.
This would empower you in ways which I do not think you have properly considered.
I do not want to pay to win, I want to pay to save my precious time. Once you have a full team of 141 characters you are pretty much set, having a few more does not make you considerably more powerful, it just makes you more diverse.
I have a roster now of 141 Spiderman, 141 Hulk, 141 IM40, 115 Rags, 106 Magneto, 96 Punisher, 93 GSBW etc. I finished 4th on the Avengers Elite Tournament and have spent money buying covers at the beginning and I would love to spend more money on the game, but the cost of leveling up a character using ISO is insulting. I have never bought ISO as the exchange rate makes no sense unless you are using a stolen credit card to buy it.
Think about this, I already have a full roster, every cover is maxed out with the exception of the 4*s. I would be HAPPY to pay to max out a cover to play around with it and probably shelf it in favor of whomever my favorites are. I have money, what I do not have in great abundance is time.
These upcoming nerfs to Spiderman and Magneto are really bothering me as I have invested so much of my precious commodity (time) grinding ISO that it makes me a bit sick to think about them being made irrelevant. If you made the leveling less expensive many people would pony up some cash at a reasonable rate and max out toons left and right. This would also lessen the sting of balancing as alternatives could be quickly brought into play, but as it stands now, when you guys screw up my level 141 characters it represents a HUGE investment on my part, it is not trivial.
Here is the thing, you release a new character on a monthly basis or better, I would be happy to play the hell out of the tournaments to get the covers, then I would be just a glad to pay to max out the character rather than grind for weeks on tournaments that I do not need or care about. If you charged a reasonable fee to max out a character ($10) you would be making some serious cash from the Whales who have already invested in the game, its a great way to keep them paying.
The reality of this from my perspective is that the demographics of people playing games is very diverse, the ones you want money from are willing to pay for one thing more than any and that is time. If I could pay to insta win the game it would not be fun, but if I could pay to save grind time I would do it gladly as I have other things in life to do, like make money.