The economic impact of having a good sized university in a city is incredible. There's a built-in tax base that doesn't need jobs, but has disposable income to buy stuff that gets taxed. The jobs that are created are significant, but the money spent by the students is the real kicker. You think Boulder would be what it is without CU? Not even close. Remember Boulder's history - it was a supply town for the mines up in Nederland and Caribou. Once those mines closed down, Boulder would have been nothing had it not had the University. You think NCAR, IBM and the bureau of standards would be in Boulder if CU wasn't there? Same thing goes for Ft. Collins. That town would be nothing if not for CSU. It wouldn't have the industry it has. It wouldn't have the tax base or the population. They owe their very existence to that university.
This multiplier effect is huge. The student money supports a large base of small business that creates an environment of creativity businesswise. Also huge is the "Brain power" impact that comes from highly educated individuals who work at the university also creating or working in the private sector as well as university graduates who like the town/city well enough to stay or return as they build their businesses.
Sacky mentions NCAR, and the NBOS, in Boulder and you could add NCAR and some other governmental enterprises. Also IBM and you could add Ball and some other major corporations but to that add the small to medium size high tech and research businesses that are in Boulder. I would bet that most of them have a CU connection in terms of founders and/or management.
This all could be translated to Ft. Collins as well.
Despite all this I am not convinced that the cost of building a new stadium for CSU is justified. It would be better than what they have but that better isn't going to be millions of dollars better, they will still be CSU on the field no matter where that field is and they will still struggle to be relevant.