The brain uses something like 1/5th of the oxygen you take in, what is it doing with all that?
If we use the phrase "cost benefit analysis" in a general sense (rather than the specific activity [1]) for coming up with the "best" thing to do then I think it's practically tautological that your brain is a gland for just that.
> if that were the case, under the presumption that we have some self-determination, it wouldn't tell us much about whether its morally good or not or aesthetically pleasing or whatever criteria we might use.
From what might be called an "evolutionary fundamentalist" POV "self-determination" is also something your CBA-gland does to get your genes into the future and (perhaps paradoxically) you don't get a choice about using it to maximize your expected future benefit.
[1] https://en.wikipedia.org/wiki/Cost%E2%80%93benefit_analysis
There is no evolutionary reason to believe that literally all your brain power is devoted to cost benefit analysis, and even if that were the case, under the presumption that we have some self-determination, it wouldn't tell us much about whether its morally good or not or aesthetically pleasing or whatever criteria we might use.