I have a 30 on the ACT, and I'm debating whether or not I should retake. Money is tight, so I don't want to register for the test if it's unnecessary. The schools I'm worried about are UC Irvine/UC Davis, UCLA, and Berkeley. I do have a high GPA (3.9 unweighted) and have taken 8 AP classes, so I think my other academic qualifications fit the bill. Any advice?
If you have the resources and feel confident that you can improve your score, I would recommend retaking the test. Your score is currently at or slightly above the average ACT scores of admitted students at each of the schools you're interested in attending. Here are the average scores:
UC Berkeley: 30
UC Davis: 27
UCLA: 28
UC Irvine: 27
You can see from these numbers that a 30 won't put you at a disadvantage, but it also won't improve your chances by itself. It will have a neutral effect on your odds of admission.
If it's going to be a struggle to pay for the test, and you don't have a fee waiver, focus on other aspects of your application instead of retaking the ACT. However, since you still have room to improve and really impress these schools, retaking the test would be to your advantage. Look into getting a fee waiver for registration. If you manage to get one, retaking the test will be a win-win option.