1 On average, a PO box is going to cost anywhere from $2 for a small box in a rural area to as much as $35 per month for the largest box in a metropolitan area such as New York City.2  To get an exact price for what your local post office is charging, this USPS.gov page can give you the exact figures from each city. To receive a PO box at a local post office, they are going to ask for two forms of valid I.D. 2  Pieces of identifications can include your driver’s license, passport, insurance cards, voters registration, college I.D. and more. 3  Some post offices may not have PO boxes available.