In my experience the cheapest convenience store bentos in japan are roughly equivalent to slightly better than fast food Japanese in California.
And usually the price is much different too. Like 500 yen, which in America might be $4.50 or so, but the equivalent in America (or California at least) would cost maybe 3x as much.
I joke about it with a Japanese friend I made on one of my trips. There's a Japanese family restaurant chain that has some locations in California (Yayoi). In japan it's just another family restaurant where you can get reasonably priced meals (maybe up to 1500 yen but I haven't been in a few years) and in America the same restaurant easily charges $20-25 for regular meals.
Food tends to be very economical in Japan, at least to me as a visitor.