I'm not ignoring the payouts of other bowls. I realize that all bowls don't pay $15mil like BCS bowls. But I also don't have time nor the desire to look or put a pen to potential payouts for potential Big 12 tie-ins.
Did TCU add anything in terms of TV market money for the Big 12? They already had the Metroplex on lockdown.
That's a bit of a different case, though.
When the Big 12 was looking to add a team at that point (which following not just the NEB/CU departures but A&M's as well):
1) they were doing it for conference survival and to make sure they didn't lose their television contracts,
2) the networks that had their contracts (FOX, ESPN) said they wouldn't get rid of/ change those those as long as the Big 12 had at least 10 teams,
and 3) when Neb and CU left the year before, the networks decided/let the Big 12 know they weren't going to reduce their television contract amounts (they would still keep paying the conference the amount it had previously agreed upon; 10 teams would now be receiving the same amount they had previously agreed to pay for 12 teams, or about 20% more each), likely to provide either some degree of stability or prevent further leaving that offseason.
There wasn't as drastic of a monetary/market consideration involved (or at least, a future one) when they were making this decision.
If CSU can remain competitive you can get the Denver market back and make BYU an offer they can't refuse and you grab a decent market in the West. It would be similar to ND but obviously on a smaller scale.
CSU isn't going to get you Denver, unfortunately. No more than NC State or ECU would get you North Carolina.
And - unfortunately for both sides: BYU doesn't want to drop the TV contract ESPN just gave them...so it'd take a pretty massive one, likely much more than they're actually worth; while for whatever reason, also, the Big 12 doesn't really want to touch/do much with BYU, for some reason.
Plus, BYU doesn't really give them terribly much a market in the other western states...maybe some of Utah?
It wouldn't really be like a smaller scale ND though.