It would be hard to quantify the return on investment to the taxpayers, but free quality higher education must have contributed to the thriving California economy and culture.
I collected some numbers on that a few years ago: http://www.kmjn.org/misc/uc_funding.txt. That table shows total and per-student support; the latter is partly impacted by the growth in student numbers. But if you convert the numbers for each year to percent of state GDP, or support per resident, there's a large decline in those terms as well (in GDP terms, from roughly 0.3% to 0.1%).
Interestingly there's no real correlation between partisan politics and the numbers. Funding was generally high under both Republican and Democratic administrations in earlier years, and has been cut under both administrations in more recent years. My guess is that as a "discretionary" line in the budget the UC is just relatively easy to chop, when someone making up a budget needs to close some holes, so it ended up bearing the brunt of the recent budget crisis that was caused by other parts of the budget.
More like, it came down to cutting education, cutting pensions, or cutting healthcare spending. This is a problem across the entire country, but California has more generous pension and healthcare promises, so the problem is especially acute there.
When my province came out of what we call the Révolution tranquille (Quiet Revolution), we had one of the lowest postsecondary education population rate in Canada. By creating a bunch of public universities with extremely low tuition fee, we have a whole different picture 40 years later with a well educated population.
Without assuming too much, I'm not certain when you went to school, based on those numbers it seems a while ago.
2013-2014, average in-state tuition for 4-year public universities is just over 5,000 per semester, and that DOESN'T include housing, meals, books and other incidentals - it assumes you only pay for classes, nothing else.
Wyoming was the lowest average in 2013-14, at $4,404 for the year (2202 per semester), and under $9,000 when you include housing and meal plan - not bad, really.
Anymore, even with Pell, state grants and federal subsidized loans, students still have to come up with 1,000-3,000 per semester. So, that's where the unsubsidized and parent plus loans come in.
Source - my own institutional research at a community college for the last two years.
For comparison, U of T in Ontario comes to around $6,000 p.a., or $13,000 for Engineering.
For example, land grant colleges were set up so that farming profits paid for the next generation's education. We've largely replaced that with student-loan debt based education, which sets graduates back for years or even decades. Now we have a stagnant economy where most people are struggling to get by instead of making meaningful contributions that increase their wealth and ability to pay/donate down the road. The founding fathers knew why public education was so important, yet at every turn we see people constantly attempting to privatize and marginalize it. It frankly disgusts me, which I know I’m not supposed to say on HN, but there it is.