If you will be at the festival for all 3 days, then a nights & weekend parking permit for UCSB will save you $4.
It costs $8 for 2 weeks. The daily rate is $4.
I think that it can only be purchased online. Here's the link:
https://tps-ucsb.t2hosted.com/cmn/auth.aspxThe process is annoying on a computer, but much worse on a phone.
-Matthew