I don't think produce prices are all that competitive. For one thing, it's terribly difficult to comparison shop them. The prices change all the time and you pretty much have to visit the store in person to find out what they are on any given week, so it's either waste a lot of time shopping around, or go with what you find in the store you usually visit anyway.

The fact that the prices are usually round numbers (e.g. when I buy apples they're almost always $0.99, $1.49, or $1.99/pound) seems to support the idea that stores aren't trying hard to wring out every last bit of competitiveness from this stuff.