What I can't figure out is why this has to be done in a single monolithic project with a ridiculous price tag. I'd approach the problem as a series of steps that start with the decision to save it.
Step one following that would be to stabilize the situation and replace the dilapidated exterior so that it ceases to be an eyesore. How much would it cost to simply re-do the exterior in the same smooth silver material that covers Reliant Center and parts of Reliant Stadium?
This stuff
Eyesore gone. Attractive building at the center of Reliant Park that no one can complain about accomplished.
Now for step 2 focus on interior demolition. Remove the seating and in fact most everything inside the shell as every plan mentioned to date has suggested. No one outside the Astrodome would even know there was work going on inside. Once the seating is removed you take as much of the interior floor of the dome as possible down to the playing surface level 35' below grade.
How much does it cost to design and build a parking garage? Constructing it inside the Astrodome would at least have the advantage of never being slowed by weather. 2 stories? 3 stories? put enough in to bring the new "floor" of the dome up to roughly ground level.
Inside underground parking for 18-21 acres of cars would remove a lot of cars from the surface lots that are not involved in tailgating. That would free up even more space for tailgaters so don't just think of money made from people who would be parking there. Think of the money people would pay to get more room to tailgate. Think of what you could put in space freed up by those cars.
8-10 games a year (plus playoffs) the party just got
bigger and at the same time more convenient for people who just want to get in and go to the game. More people means more money even if they are just staying in the parking lot tailgating all day.
Now you have an empty shell that's got a flat floor with 2-3 levels of parking underneath. It's climate controlled, it's not unattractive from outside, and it contributes something that wasn't there before to offset its expense.
Put something in it. Call it expanded Reliant Center space and put nothing in it if you like. At this point there's no need to rush to decide what to do with it. It can be used for events as is.