But it's been reported in some places that the money is going to be close on the next contract - which honestly lines up with the data more than thinking the Big 12 will have a huge advantage there.

Also, the PAC12 can go to a Big 12 team with a agreed-upon contract in hand from ESPN saying "here is exactly how much you'd be making on a new TV rights deal" and that number is absolutely going to be more than the Big 12 is (currently) offering. Despite losing their most valuable programs, both the PAC12 and Big 12 will be making more per year on this next deal than on their previous - live sports has just become THAT much more valuable.

And yes, the Big 12 can counter by saying - "well we reasonably project that you will probably make slightly more with us if you don't leave and we add some of their schools", but they won't have the same hard agreed-upon numbers, because they're not in actual negotiations yet.

This is an advantage and disadvantage for the PAC12 in that they negotiate first.
The current Big 12 deal (signed in 2012) pays $20M/school/yr for Tier 1/2 rights. The current PAC12 deal (signed in 2012) pays $21M/school/yr for Tier 1/2 rights.

Both have lost a lot since then (Texas A&M, Mizzou, Texas, Oklahoma from the Big 12; USC, UCLA from the PAC12). But if the PAC12 can go and show conclusively that they'll get $32M/school/yr if they add (for example) Ok St, BYU, TCU, Texas Tech, it's going to be tough for those schools to launch into the unknown of waiting a year to see what sort of deal the Big 12 can put together without any marquee brands. There's value in knowing where you stand instead of being left to the future inclination of a network.