SMU wanted out of the American, and was willing to pay to do it. That Oil/RE money is long enough to where they can eat the lack of TV revenue, if it means that once that period passes, they are getting significantly more than they did in the AAC.
I'll never get why the Big XII didn't take SMU in their last expansion, they are a perfect fit. I think that underneath it all, schools like Oklahoma State, Kansas State, etc. know that SMU in the Big XII would wipe the floor with them. I'm the son of an SMU alum, grew up around the program, they've been prepping for this day for 30+ years. I'm happy we are getting SMU at a discount in the ACC, because it adds another city school, a school in an area we recruit well from, and it's a school that is going to be firmly in the "Football School" camp. If the ACC starts falling apart once FSU, Clemson, and Miami bails, don't be shocked if SMU finally gets the call from the Big XII. It's a lot easier to sell SMU coming from another major conference, compared to the AAC.