For example, the University of California system was budgeted about $2.1 billion in state money in 1990, which is $4.3 billion in 2020 dollars. At the time, there were 9 campuses, serving 125,000 undergraduates, in a state with population 30 million. Fast-forward to 2020, and the system is budgeted $3.9 billion in state funds, which have to cover 10 campuses, serving 225,000 undergraduates, in a state with population 39.5 million.
If state funding had kept pace solely with inflation and state population, not taking into account any above-inflation cost increases, or the increase in percentage of high-school students who go to college, the budget in 2020 ought to be $5.6 billion instead of $3.9 billion, about 45% higher. If it had kept pace with both the number of students and inflation (but still no cost increases), it'd be $7.7 billion, or almost double.
My read of why it's been cut isn't that CA politicians really think the UC isn't worth funding in 2020 like it was in 1990, but more that it's been a victim of convenience in fiscal crises. CA has had a few state budget crunches over the past 30 years, and the annual UC budget is "discretionary spending" and not locked in, so is procedurally easy to cut. It was slashed significantly during both the dot-com crash and the 2008-2012 budget crisis, leaving us with today's lower level of support.