Just a quick analysis of the gold it might take to level up all your abilities for 75 heroes to go from Level 70 to Level 80.
DB tends to keep things pretty linear in terms of stats when going from level to level. This is true in terms of hero stats, like Attack, Defense, etc. It is also true in terms of the gold required to increase ability levels. Just for background, ability gold increases from level 1 to level 60 were a flat 500 increase for each level. There was a jump to 1000 per level from level 61 to level 70.
So, I would anticipate there might be another jump when going from level 70 to level 71, and that that same increase would apply from level 71 to level 80. My analysis uses a 1000 increase per level as a baseline (basically the same increase we saw from level 61 to level 70). It also includes sensitivity analyses using assumptions of 1500, 2000, 2500 and 3000 per level. My best guess is that since they doubled the amount going from 60 to 61, they may also double it again to 2000 when going from 70 to 71. No way to know, so I'm including several different assumptions.
The bottom line is that my bests guess is that it will take from 135 million to 168 million in gold to increase the abilities for 75 heroes from level 70 to level 80. As always, if there is a flaw in my analysis, I welcome the feedback. Thanks!