I never know if people actually do want to know, but ...
The reason why produce is cheaper at Asian stores is typically a combination of two different factors: where in the store the profit is, and how the store chooses to make the compromise between spending money on making it seem nice to be at versus taking the money and running.
Mainstream grocery stores make their profit on prepared foods and produce. This is a consequence of a lot of trends, but one has been that eating fresh produce has gotten a [possibly not entirely deserved] reputation as being both the healthier and also the more classy thing to do over the past 20 years. [Meanwhile, canned and frozen veggies have gotten substantially cheaper, even tomatoes, which can beautifully.]
Last winter, Mark Bittman (the NYT's excellent food columnist, whose How to Cook Everything
is the cookbook of the 1990s) did a funny column that basically summarized to "Gosh! Out of season "fresh" produce sucks compared to the good quality frozen stuff, and the frozen is much cheaper, too! I wonder why we all buy fresh nowadays? Could it be marketing?
So demand for fresh produce among white middle-class folk is high enough that stores can raise prices on that with minimal difficulty, making nice profits, and then have lower prices on milk to advertise.
This is different for Asian stores; many of their customers are most excited about getting the right kind of the staples from back home, or are white folks who have no idea what Asian staples ought to cost; moreover, if you live in a part of the world without Weggies
, they may have a near-monopoly on those products. So you wind up with expensive rice, or mirin, or tamarind paste or whatever, but much of the produce is pretty cheap; that can even prompt folks to come there and then spend a lot on something else. Often this sort of situation lasts until the community gets really large, and a huge Asian megastore like the Uwajimayas in Seattle, or Super-88 in Boston or T&T's in Toronto moves in, but even then it may not have a completely market-changing effect, since lots of folks aren't willing to drive to the other side of town to buy their $20 in Asian staples.
And then the other factor just comes from how much money the store pays in general for its produce or for store upkeep: one local store here advertises that it gets produce deliveries 7 days a week. Well, that's fine, but I don't suppose they're the one picking up the extra tab...