If he goes to the Dodgers (which he won't) then his value would drop some. I actually think his value would remain about the same at Petco, the stadium won't be nearly as bad later into the summer. Balls will be flying out of there pretty well.
As a Brian Giles owner, I hate to disagree, but, San Diego is a pretty temperate place. The weather doesn't change much at all, as they don't exactly have "seasons." It's usually low 80s year round. I don't think anything's going to change ball park wise.