The Post, Shankar Vedantam, reported yesterday that immigration cuts gas and grocery prices. The idea being, we natives get lazy and patronize the nearest gas station and grocery regardless of their prices. Immigrants, being more energetic and more price conscious, shop around. The shopping around is the element of competition that tends to drive prices lower.
(I think I've mentioned before a supermarket location that went through 3 incarnations, being too small for efficient Safeway/Giant operation. Now it's been open as a Latino themed market, with good vegetables at lower prices than our regular Safeway.)