Hi Larabell,
I've never shopped at a co-op in Japan before, but their prices must be cheaper than your regular grocer, because where I bought the watermelon (on July 3rd of this year), there were no 500 yen whole watermelons. The one I bought was the largest size they had, so yes, the smaller watermelon was 1400 yen as opposed to 1800. And this was a stand alone supermarket far from any train station/ department store in Fujisawa. And all the years I've lived in Japan, I don't remember seeing any bagged apples. I guess you're lucky and know where to shop.
As for eating the local diet, as much as I like rice, rice for breakfast seems to give me heartburn, and requires okazu of some sort, plates to put it all on, a pair of chopsticks, and you have to open the refrigerator. Bread, you can stick in the toaster, slap some peanut butter on it, and stuff it in your pocket or backpack for eating on your way to the station.

Do any of the expats eat rice for breakfast? What do you think about bread in Japan?
I had trouble finding good European style al dente bread. Regular bread seemed to be sweet and too soft.

Mari