I have tried searching it but it gives following results:

• higher taxes? - In my country taxes are same everywhere

• rent? - In cities rents are higher, because more people are on reach of shop. So owner pays more in rent to sell more goods than when he would sell them in worse location otherwise. He makes bigger profit in return.

• higher salaries? - People have more money. People buy more things. Thing get pricer in cities. Cost of manufacturing product doesnt depend on location of sell. Factories adjust by sending more products to cities instead of less dense location. Market balances and price is same everywhere. (A bit higher althought)

• transportation from factory to city? - I would say its opposite due to economics of scale. Train transports products to distribution centres around city. Inside city delivery truck travels less distance to supply more shops. On the other side truck has to ride much longer distance to supply less shops.

• higher demand? - People in cities consume more? Same story as with salaries apply here.

• perceived value?! - Maybe I have started to look for answers too hard

• weather?! - This could make sense in some cases like heat island and cold products