Unless you buy your fruit, veg etc. from local shops or vendors, and familiarise yourself with the actual cost of all items before you purchase them, then you can't expect to buy anything cheaply as most foreigners in Tay Ho live and work in Hanoi and enjoy huge salaries compared with the wages of local people. Hence the high prices in that minimart, which have nothing to do with being a tourist trap. You should also bear in mind that anything imported is subject to import tax and undoubtedly costs more than it does in LA.
In any event, nearly all supermarkets carry only a limited range of fresh goods and meat because local people prefer to purchase from street markets or vendors.
The largest supermarket near the Old Quarter is the two-storeyed Intimex on the western side of Hoan Kiem Lake, which has a sizeable range of frozen meat and fish, as well as bottled and canned staples etc. It's near the corner of Le Thai To and Hang Trong, and is set back from the road down a short alley. You'll have to leave your shopping bag or rucksack in one the lockers situated in a room off the alley.