The going rate for a (superb) lunchtruck sandwich in Philadelphia is <$5. The standard non-deluxe hoagies are usually <$3. Breakfast style hoagies (egg based, not meat) are usually $2.50.

Same sandwich from a corner deli (cheapest brick and mortar place that you will find a sandwich) is $5-$8. The competitive advantage that corner shops have is that they will, if they are clever enough to make themselves a "restaurant", sell beer.

Lunch trucks are not "cheap for NYC of SV", they are just cheap.

Furthermore, lunchtrucks do great business in Philadelphia during the brutally hot summers and icy winters because they are cleaver enough to part places where where they are the option that involves the lease amount of time walking outside.