Seiyu is a subsidiary of Walmart. They're not super-close to my home, but I have been making the trip every other month or so, because they have relatively low prices on a few stockable items like frozen broccoli and gyoza, and they have some inexpensive potato chips in American flavors that I miss. It's hard to get cheddar flavored snacks here that aren't also sweet.
To get an idea of prices, think of 1 yen as 1 penny (meaning ¥100 is around $1.00). The exchange rate isn't 1:1, but in general, that's a good guideline for thinking about food prices, considering the relative cost of other life necessities.