When I go downtown, I always make sure to have a few rolls of quarters in the car. You can do e-payments but if you need to "leave no trace" quarters are the way to go.
As Kant said it's $2.50 an hour. Street parking within a few blocks in the city is usually easy enough to find if you don't mind walking a little bit. Always nice to save money, this hobby is expensive, and it adds up over time.
One piece of advice. If you use meters, make sure you put a little extra time in (i.e. I wouldn't do 1 hr exactly because if shit is delayed or runs over, they will ticket you. And they are vicious. One time I got a ticket on my car and I got back to my spot 4 min after time ran out!).
Last piece of advice. Philly uses a "zone system" for parking and the kiosks are fairly easy to find, however, and this is CRITICAL... sometimes on the same street, each side of the street may be in a diff zone. So if you park street parking, don't cross the street to feed the meter. Try to get a meter on your side of the street. Alternatively, make sure you see the sign with the zone # for the side you are on, take a pic with your phone, go to any kiosk and select "Parked in another zone" and you can enter the zone number.
This is critical, because if you just feed any random meter and it happens to not be the zone you are in, you will get a ticket.