You probably want to check into some charter schools such as Julian or Dehesa. They may allow you to take dual enrollment for free. Be aware, because of the budget cuts, it is very difficult to get classes. Being that you haven't even registered, the chances of you getting any classes for the fall (which starts in two weeks from yesterday) is very slim. You might get the ball rolling, though.
What area of San Diego are you in? There's also City, Mesa, and Miramar, which are all a part of the San Diego Community College District. They are all in the city of San Diego. There's Palomar in North County which is quite friendly to homeschoolers. Palomar, I believe, doesn't charge for high schoolers but again, classes are impacted and hard to get into.
Another option is auditing classes at local universities (audit meaning for free).
My son's done a mix of CC and audited classes. We could never afford university tuition.