In the town closest to central america.

The labor to produce, ship, and shelve the banana determine it's cost along with whatever margin the store that sells the banana is willing to take. Walmart, for example, could be perfectly willing to sell a banana at a loss if they think that will get you in to buy a TV.

This is why dollar stores exist and often kill off local grocers. They can sell a lot of non-perishable goods at a loss and win back by understaffing the location and overcharging on non-perishable goods.

I live in a city with probably around 50 different clinics, but they are all associated with 3 major medical groups. It isn't a lack of buildings that's preventing competition.