Total ISO-8 required for leveling up

Ok, so I've gathered up a bunch of characters and I'm trying to figure out if there's any rhyme or reason to how much iso-8 is required to level a character up to max level.

For 1 star characters, I have enough data to conclude that the cost to level up is a standard 20 extra iso-8 each level. So you need 100 to get to level 2, 120 to get to 3, 140 to get to 4, and so on and so forth. This means that to get a one star all the way to level 40 is takes a total of 18,720 iso-8. For Iron Man Model 35 which has a max level of 50, he follows the same formula and requires a total of 28,420 to get to max level.

But the 2 star and 3 star characters don't seem to have a discernible pattern that I can see (albeit I have much less data to work from there). Has anyone else tracked, monitored, calculated this? Also do the max level 69 2 stars and the max level 115 3 stars follow the same pattern (like Iron Man Model 35 does) or is it different?

Thanks!