Is there really some kind of nation wide grape price standard?
Not to my knowledge, no.
Don’t they have to add costs for transportation to places where they do not grow grapes?
Certainly, but with economies of scale and good logistics, the prices across the country aren’t that much different. There are some exceptions, such as pockets of the continental USA that are under served, or states like Alaska and Hawaii which don’t benefit from the continental USA logistics. That was the “geography” question I had to the poster.
The prices of grapes at my grocery stores right now are $2.99/lbs and they are imported from Chile or Argentina usually because grapes are out-of-season for the USA right now.