There's never a charge for self-parking at the Disney resorts. The guard at the entrance to the Poly will ask for a photo ID, but you'll likely not have a problem parking there without having an ADR. They will sometimes issue a 3-hour parking pass if it's a busy time of year.
I should add that at very busy times of year, the guards can be a bit stricter about allowing entry to the lots at the monorail resorts. On the unlikely chance that you're turned away from the Poly lot, you can park at the TTC (the Magic Kingdom's lot) and walk from there to the Poly. You will be charged the parking fee for the MK lot if you do park there.