In his comment, Ahniwa describes searching for a bit, then getting to California Department of Education and browsing to the "Student Expense" page: (http://www.cde.ca.gov/ds/fd/ec/currentexpense.asp ). It's an interesting page as it tells you that cost/student is measured in terms of "Average Daily Attendance" (ADA).
Unfortunately, that page only goes back as far as 1998--but it's a pretty good solution!
If we want to go back further in time, the only way I know to get this data is through the Rand Corporation's. I found this factoid out by doing the query:
[ california annual per-pupil spending data set ] - since I was looking for the complete data, it was pretty easy to see that only Rand had all the data compiled together.
BUT... once you click through to Rand's data set page http://ca.rand.org/stats/education/perpupil.html -- you quickly discover that you need to login to actually get the data. Unfortunately, logins start at $270 / person.
Luckily, I happen to know that the Rand data sets are often made available through public libraries! (Keep this in mind: this is true for MANY data resources that otherwise cost a lot of money.)
So I connect to my local library's page (in my case, the Palo Alto library lists all of its online resources on a web page, which makes it easy to find).
All I did was then click through to the Rand database site for California Education Statistics, login via the interstitial page (requires only my Palo Alto library card), and voila! -- I'm into the Rand dataset.
Once there, it's an easy navigate (Databases>Annual spending per pupil) to the list of counties of California, and then a quick download of the data from 2000-2009 as a TSV file (easy to then import into your favorite spreadsheet program).
Here's an example chart from the data. You can easily see the bug in the data (no, the Montebello school district did NOT spend $47K / student).
The moral of this particular story?
Sometimes you still need to find out where the data is kept... and that the access path might be through your public library!