Great value. Supermarket explains it. Like most tourists to Japan, I went to convenience stores more than supermarkets but when I went to one I was amazed that it was even cheaper (I know it's obvious but I didn't expect it given how reasonably priced convenience stores were)! Then there are also those shops that specialise in cheap bento. You get a decent meal for 500 yen or a really good and big one for 1000.