NPR has been crunching pizza numbers today and yesterday. Today they layout how much pizza costs by neighborhood in five cities, including Philadelphia. The data comes from Grubhub/Seamless and interestingly, Graduate Hospital is the most expensive neighborhood for a cheese pizza. An average pizza in G-Ho comes in just under $15. Elmwood and Oak Lane are the cheapest neighborhoods for pizza with a plain pie being available for under $8. Of course with this data coming from Grubhub and Seamless, places like Osteria, Pizzeria Beddia and many others are not included.
As for the most expensive pizza in the country, Midtown Manhattan is getting $23 for a plain pizza.