I suspect it is to do with the amount of pedestrian traffic passing through an area. When you have a high population density there is an increased amount of foot traffic in the area you can charge less per individual serving because you have a higher overall volume of traffic.

Where I live in Australia the cheapest food tends to be Kebabs which congregate around pubs. There is a high amount of students walking (stumbling) home after a night out etc so they can afford to be cheap since they get so much foot traffic coming through.