here is the report, where the 21 billion number is coming from.
http://www.fairus.org/site/DocServer/USCostStudy_2010.pdf?docID=4921
It breaks thing down into federal and state/local. So a comparison against just the state budget is improper; you must compare against the combined budget of state and all local governments. Los Angeles County's budget is 23.6 Billion for ~9-10 million of the ~36 million in california, so it should be easy to assume the total state/local spend is around 180 to 200 billion(note: this is a ballpark estimate if you have better numbers I will defer to it). So 21 billion of 200 billion is a little over 10% and the the illegal population is a little under 10%.
The report made the following assumption to "inflate" the numbers:
In our cost estimates we also include the minor children of illegal aliens born in the United States. That adds another 3.4 million children to the 1.3 million children who are illegal aliens themselves.We include these U.S. citizen children of illegal aliens because the fiscal outlays for them are a direct result of the illegal migration that led to their U.S. birth.
Since education is the largest cost of illegal aliens, nearly quadrupling the number of kids greatly increases the total cost. Education costs are 11 billion for illegals in california, but it is really ~3 billion for illegal alien children and ~8 billion for US citizens children of illegal aliens.