I'm a singleton. I probably spend about $50-$75 per week. I know I could spend less, but I like getting fresh fish from the market, fresh fruit and vegetables, good cuts of meat, etc. (I live about a block away from one of the bigger markets in Toronto, so I rarely go to the actual grocery store.) I also buy wine and vodka. I don't see how that's a low amount. What I buy is fairly expensive, all things considered. Maybe you're buying a lot of frozen food? That stuff can add up. It can actually be cheaper to buy fresh stuff, make a bunch of meals all at one, and freeze them into portions. This is what I do.
Before, when I actually ate the work lunches and dinners that are provided to me, I spent a lot less. But I was a lot fatter. Stupid free lasagna.

Also, when I was in university, I could easily live on $15-$20 a week, but I ate a lot of ramen and peanut butter sandwiches.
Some hints I can think of offhand:
-- Don't buy bagged vegetables. It's way cheaper to just buy the whole ones and take the small amount to chop them up for yourself
-- Buy whole fish if you're not squeamish about filleting them yourself (also, cooking them whole improves the flavour, but some people don't like eating food with a head)
-- Buy in-season vegetables and fruit
-- When something is on sale, buy lots, e.g. I love the Campbell's vegetable soups in a box and they were on cheap a few weeks ago, now I have like 12 boxes in my pantry
-- Don't buy diet snack food unless you super-need it. It's very expensive for what it is. I couldn't believe the costs of those 100-calorie packs of snacks, it seems just crazy to me!