Any ideas what is the combined contribution of GPA and SAT in UCs' decision?

Still what is not clear to me is the combined contribution of GPA and SAT in UCs’ decision.

For example:
Case 1: UC GPA:4 with SAT 1300
Case 2: UC GPA:4 with SAT 1400
Case 3: UC GPA:4 with SAT 1500

What chances would you give for let’s say:
UCSC:
UCD:
UCSC:
UCI:
UCSD:
UCLA:
UCB:

It cannot be the same…But what difference can test score can do? This is what is not clear.
I am aware that EC’s and Essays are also taken under consideration but lets assume, in our simple example, that they are the same for all 3 cases.

GPA is king at the UCs. With a 4.0 UCGPA, you are unlikely to be admitted to UCB, UCLA and UCSD regardless of your SAT. The same is probably true for UCSB, UCI and UCD but, a high SAT could tip the scales - that said, a 1300 is
a pretty good score so, the difference in your chances vs a 1500 is only marginally better…
all 3 cases are quite likely to be accepted to UCR, UCM and UCSC.

As you note, ECs and essays are also considered so, apply to all of them, it could happen. Just be realistic with your expectations.

Also, apply to SDSU, CPSLO (depending on major) and another CSU or two so you have lots of options when the time comes.
Good luck

@Kentriko: You also have to consider intended major and how each UC admits. Some admit into the University first like UCSD/UCI then into the major or by College such as Engineering vs. College of L&S or just into the University and then you have time to declare a major like UCSC. A 4.0 GPA SAT 1300 applicant could get into UCSD but mostly likely not into a highly competitive major such as Engineering but be admitted as Undeclared and the same could hold true for a 4.0 GPA SAT 1500.

Too many variables to make a blanket statement about GPA + Test scores or GPA vs. Test scores. In general, competitive GPA along with competitive test scores is the best combination.

I would advise that you know the UC you are most interested in. Some of the UC’s score on a 50/50 pie. 50% gpa and test scores and the other 50% is holistic. (your essay, EC’s, first generation etc) I would say Davis and Merced are GPA driven. I know students who were admitted to CAL with a couple of C’s in their transcript, this one student received a request to submit an additional supplement and was accepted. With that said, like the other comments, the higher your GPA, test scores and academic rigor (compared to the other students in your school) means a lot. Another uncertainty is you just don’t know the quantity of students applying in that particular year to a certain UC, how many apply to the major you are interested etc.
Each year at the UC counselor conference the numbers are daunting, but it helps when you have another person who knows what they are doing helping you. If you have a college counselor at school, go to them, have them check your work and take proper tests and classed if needed. I’ve had students rejected because of simple errors.