Schools would likely see an ROI if they could get out of the ACC and into the B10/SEC/B12, but I think you're total figure is low and you skipped some issues that will have to be resolved or bought out.
So let's say they decide to do it. First order or business is the ACC buyout: $120M each.
Next, the left behind schools will probably lose out about $10m per year if their new TV deal is in the ballpark of what the AAC gets. So that's $400m, or another $40m from each school leaving. $160M
Now here's where things start to get interesting.
First, if the ACC sticks together, it's still a P4/5 conference and will get CFP money accordingly. How much? Really hard to say at this point. Suffice to say, it'll be a whole lot more than they'll get as a non-power conference member. Will they just give this up? No they will not. They currently get like $5M, but the next deal should double that. Another $40M each! $200M
Next, if the ACC sticks together, those schools get the prestige of being the P4/5 conference. How much is that worth? This one is harder to quantify, but maybe $4M per year? $240M.
Here's one that's tougher yet. Notre Dame. First, they get about $10M a year from the ACC for their non-football sports + 5 FB games. The ACC solves a lot of problems they'd otherwise have in keeping their FB independent. Money's not the most important thing for ND, and ESPN/Fox have no sway with them (assuming they re-up with NBC). Maybe they agree for $30M for each of the 10 years? $30M each, $270M total.
So the B12 doesn't work as a destination. $32M * 10 = $320M- $270M = $50M. All that work and negotiating to come out with $120M less than if you did nothing. If the can get out for $120M, makes sense. If the B10/SEC 6 blow up the ACC, even better. Otherwise? They're stuck.
So some of the buying out gets shifted to the B10/SEC schools. I think there's six of them (UNC, UVA, Miami, FSU, Clemson and either NC State or VA Tech, but not both). Leaving everything else the same (and assuming the 4 the B12 picks up just kind of let it all slide since they'll be better off if they can get out for free/just the $120M exit fee), those six would have to shoulder about $300M each. Their break-even point is at ~$47M per year for the TV portion. Doable, but not exactly massive after all the up-front investment.
Still unanswered: Where do these schools get the $270M+ needed up front to make this happen?